First I would like to thank you for bein patient with me. After some contemplation, I think I've identified what aspect of my argument hasn't been properly justified, which causes this kind of discussion.

Let's first define C as the set of all algorithms that are computable by any Turing complete system.

The main attractive feature of Turing Completeness is specifically this universality. You can take an algorithm running on one Turing Complete system and port it to another, with some amount of translation work (often just a compiler)

Now let's define the subset of all algorithms C that we are not to properly parallelize, let's label it U. (U is a subset of C).

The complementary subset of C, that CAN be parallelized properly we label P (P is also a subset of C).

Now define algorithms that require a lot of compute (>= 10^20 steps or so) as L. L is also a subset of C.

The complementary ("small" computations) can be labelled S (< 10^20 steps, though the exact cutoff is a bit arbitrary).

Now we define the intersections of S, L, U, P:

(Edit: changed union to intersect)

S_P (intersect of S and P)

L_P (intersect of L and P)

S_U (intersect of S and U)

L_U (intersect of L and U)

For S_P and S_U, the advantages of Turing Completeness remains. L_U is going to be hard to compute on any Turing Complete system.

(Edit: The mistake in my earlier argument was to focus on L_U. L_U is irrelevant for the relevance of the universality of Turing Complete systems, since no such system can run such calculations in a reasonable amount of time, anyway. To run algorithms in the L_U domain would require either some fundamental breakthrough in "single threaded" performance, Quantum Computing or some kind of magic/soul, etc)

This leaves us with L_P. There are computations/algorithms that CAN be parallelized, at least in principle. I will only discuss these from here on.

My fundamental claim is as follows:

While algorithms/computations that belong to the L_P set ARE in theory computable on any Turing Complete system, the variation in how long it takes to compute them can vary so much between different Turing Complete systems that this "universality" stops having practical relevance

For instance, let's say two computers K1 and K2 can run one such algorithm (lp_0, for instance) at the same speed. But on other algorithms (lp_1 and lp_2) the difference between how fast those system can run the computation can vary by a large factor (for instance 10^6), often in both directions.

Let's say lp_1 is 10^6 times faster on K1 than on K2, while lp_2 is 10^6 faster on K2 than on K1.

(Edit: note that lp_2 will take (10^6)^2 = 10^12 times longer than lp_1 on K1)

While both these algorithms are in theory computable on both K1 and K2, this is now of very little practical importance. You always want to run lp_1 on K1 and lp_2 on K2.

Note that I never say that either K1 or K2 are (Edit) not Turing complete. But the main attraction of Turing Completeness is now of no practical utility, namely the ability to move an algorithm from one Turing Complete system to another.

Which also means that what you really care about is not Turing Completeness at all. You care about the ability to calculate lp_1 and lp_2 within a reasonable timeframe, days to years, not decades to beyond eons.

And this is why I'm claiming that this is a paradigm shift. The Turing Completeness ideas were never wrong, they just stopped being useful in the kind of computations that are getting most of the attention now.

Instead, we're moving into a reality where computers are to an ever greater degree specialized for a single purpose, while the role of general purpose computers is fading.

And THIS is where I think my criticism of Deutsch is still accurate. Or rather, if we assume that the human brain belongs to the L_P set and strongly depend on it's hardware for doing what it is doing, this creates a strong constraint on the types of hardware that the human mind can conceivably be uploaded to.

And vice versa. While Deutsch tends to claim that humans will be able to run any computation that ASI will run in the future, I would claim that to the extent a human is able to act as Turing Complete, such computations may take more than a lifetime and possibly longer than the time until the heat death of the universe.

And I think where Deutsch goes wrong, is that he thinks that our conscious thoughts are where our real cognition is going on. My intuition is that while our internal monologue operates at around 1-100 operations per second, our subconscious is requiring in the range of gigaflops to exaflops for our minds to be able to operate in real time.