Inertia is only that - inertia. It doesn't affect the overall trend, by definition no democratic system can withstand a popular-but-harmful idea that remains popular over a sustained period of time (Brexit might be an example). If a system doesn't manifest that public opinion then it is no longer democratic.

In other words what you're arguing against here isn't rapid change, it's democracy as a whole. You can't prevent people from having stupid ideas, and on a long enough timescale any democratic system will eventually implement those ideas.

The idea of gridlock as a design objective is very harmful. The "inability to form a government" is considered a failure states in most government systems, having a government that is actually able to enact policy is a good thing.

In practice however, this gridlock does not produce any incentive to pass "things everyone really agrees on" at all, as you can see by various topics that have 90% public support (eg cannabis legalization) that have been ignored for decades. It just produces gridlock and nothing gets done at all, not even the stuff that "everyone" agrees on. This, too, is a form of un-democracy.

A lot of the "gridlock = good" comes back to people blindly falling back on the fetishism they were taught about the US constitution in grade schools - checks and balances are a good thing, who could be against checks and balances? But if there are too many checks and balances that drags the whole system to a halt - again, having a government that is able to actually enact policy without a supermajority control of all branches of government is a good thing. They only work when everyone has a "gentleman's agreement" to play nice and actually do the things that help constituents - when everyone starts playing hardball to the maximal extent permitted by law, the systems drags to a halt and stops working. And once that becomes a matter of routine as opposed to a last resort due to extremity, the systems begins to fail. That's a design flaw, that gets brushed under the rug because we're all taught "checks and balances are automatically good!" as children. Having some checks and balances is good, but too many can also be bad.

With inertia taken to an extreme you get the Polish Sejm and the liberum veto - where any single member could object and veto not just the current legislation but every bit of legislation that had been passed in that session. That certainly "slowed government down", and eventually led to the collapse of the polish government. So there clearly exists some configurations where there can be too much inertia and it leads to an inability of the state to respond to a changing world and eventual collapse. Where does the US fall? Compared to most other countries, far far on the side of too much inertia, and we are also failing to a much greater extent then most other states at the moment.

https://en.wikipedia.org/wiki/Liberum_veto

In particular I think the election of the US president separately from the legislature is very harmful. The executive and legislative agenda largely should be aligned, the cases where it's adversarial have been largely periods of "failed government" where nothing really gets passed. Of course that is also tied up with representative-level gerrymandering and state-size mismatches in the senate. The fixed 2-4-6 year cycles of the US government is also harmful compared to the (potentially) more frequent election cycles in other countries, as those countries can respond to losses in public confidence much more quickly. As such - the best solution to an unpopular president/legislature is likely a variable election cycle to get them back out quicker once they lose a mandate.

I am completely willing to trade the other party being able to accomplish more of their agenda while they're in office, for me being able to accomplish more of my agenda while I'm in office. Elections should result in a mandate to enact policy, that is a good thing. It is better to have frequent elections that result in a mandate to lead, with changes in control when the party no longer reflects the will of the people, than to have nobody able to do anything all of the time.