Below, I'll focus on model black holes, immersed in vacuum, isolated in asymptotically flat spacetime, and without regard to initial formation.

Yes, you can make such a theoretical black hole arbitrarily large, and thus geodesic deviation[1] can be arbitrarily small just outside the horizon.

The region just outside the horizon[2] is still a strange and likely bad place to be, filled with post-Newtonian effects from gravitational redshift and plunging orbits.

[aside 3]

(Astrophysical black holes must have some upper mass limit, and will generally have accretion structures that produce additional hazards).

> black hole does not imply extreme conditions

I invite you to calculate the scalar curvatures[4] in any model black hole, including one with an arbitrarily high mass (say about that of the known universe), and the geodesic equations below ~ 6(GM)/(c^2) [5]. If you do that, you'll find that extreme conditions are always manifest, and that in particular even for arbitrarily large mass model black holes, once you are past the point of no return you are inevitably drawn into a caustic, and fairly quickly by your own wristwatch-time.

Super-and-ultra-massive black holes are mostly interesting because the curvature very close to the horizon is well within the effective field theory limit of General Relativity, so theorists can be sceptical of the introduction of quantum corrections to gravitation in those regions of (all) theoretical black holes unless the corrections vanish as one takes the black hole mass to a very high limit.

> a lot of very heavy black holes begin to merge

You should consider that clusters galaxies will begin to reappear from the other side of the horizon well before they get close enough to one another to interact gravitationally. The very late time big crunch is not necessary to demonstrate the difference between switching from an expanding universe to a contracting one and switching from a growing black hole to an evaporating one.

Indeed, although late times of black hole evaporation are theoretically interesting (is there a remnant?) even the very earliest stages of evaporation are different: what rushes out from the region around the black hole is greybody radiation, not the stars, rocks, and space probes we threw in while the black hole was still growing.

Taking the mass of a black hole to an arbitrarily large value does not change this essential difference.

I don't know what you're trying to say in the final two paragraphs.

If your "I don't see why it needs introduction of new physics" is a request for information rather than a dismissal of the debate itself, you could start with the late Joe Polchinski's excellent 2013 slide deck https://www.slideshare.net/joepolchinski/firecit and the references at https://en.wikipedia.org/wiki/Firewall_%28physics%29

There is nothing virtual about Hawking radiation; it appears pretty generically close to all sorts of black holes equipped with a non-vacuum exterior, and has been shown in acoustic analogues. Unruh: http://inspirehep.net/record/775859?ln=en availabile at https://pos.sissa.it/043/039/pdf If you are perturbed off ISCO by Hawking quanta scattering off you, you likely will care very much that it is not a "virtual" phenomenon, while you are still able to care about anything at all.

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[1] We can consider the Riemann curvature tensor R^{a}{}_{bcd} u^b u^d X^c, where u^a is the 4-velocity of an object on a geodesic and X is a vector quantity in the tangent space between that geodesic and one nearby (e.g., a different mote of neutral dust in an infalling cloud, with identical 4-velocity) describing these geodesics' tendency to separate or converge. More roughly, this encodes the inwards-squash and the back-to-front stretch of this dust cloud from a macroscopic perspective. It also, in suitable coordinates, describes the spaghettification of extended bodies bound by non-gravitational forces. For spherically symmetric masses, in general X shrinks with distance from the mass. For black holes we take the mass to be highly focused so this obtains practically everywhere in the Schwarzschild interior. The interior oddities arising from breaking the symmetries of Schwarzschild with e.g. a non-negligible spin parameter do not change this statement qualitatively.

[2] In the Schwarzschild case the region R_s < r < ~ 3R_s is prone to result in inward plunges for non-massless objects, and of course the size of that region scales with M. Some details: https://hepweb.ucsd.edu/ph110b/110b_notes/node80.html Less symmetrical black hole models also have exterior regions, scaling with M, that are hazardous to any non-massless observer.

[3] The region just outside the cosmological horizon is essentially identical to the region just inside the cosmological horizon. There is an important symmetry difference: when someone else recedes past our cosmological horizon, we recede past theirs. We do not notice when we exit someone else's cosmological horizon -- we are doing it right now. We continue on diverging geodesics (cf [1]). Being on the inside of a black hole -- even one with a mass greater than the observable universe -- gives a different view. With the time one has left, one could determine (via e.g. Synge's method) that the spacelike part of the spacetime curvature is curved and thus one is inside a truly enormous vacuum black hole. If we break the vacuum condition Robertson-Walker -> Friedmann-Lemaître-Robertson-Walker & vacuum Schwarzscild -> Lemaître-Tolman-Bondi, we would not see isotropic distance-dependent gravitational redshift of luminous matter inside the LTB black hole as we do in the FLRW expanding universe: there would be an enormous anisotropy.

[4] Let's use Schwarzschild (black hole and coordinates) to avoid drowning in calculations for a black hole equipped with an "outer event horizon"; the interior metric is also much easier to reason about, and compared to cases such as Kerr-Newman, much more physically plausible. For a Schwarzschild black hole, the Kretschmann curvature scalar is R_{\mu\nu\lambda\rho} R^{\mu\nu\lambda\rho} = \frac{48M^2}{r^6}, where R is the Riemann curvature tensor. Coupled with the appearance at r = 2M of the Schwarzschild horizon, we can see that as we take M -> \infty your point about "benig[n] ... shredding-and-tearing-wise" holds. However, calculating for the interior, we find that the Kretschmann scalar explodes and becomes irregular at r = 0. This is diagnostic of a gravitational singularity.

[5] For example, https://en.wikipedia.org/wiki/Schwarzschild_geodesics#Geodes... although I'd want to resort to a textbook.