The local maximum of "fix it but don't improve it" is a problem, and the requirement for major reconstruction to be done to current codes is foundational to almost all building codes. Note that small repairs and routine maintenance don't activate this requirement - it's activated when major work is being done, and there is a percentage of total project cost cap to the requirements.

If the new local maximum is "well, we won't fix it at all then" - the usual building code solution is property maintenance codes that mandate repairs.

The buildings often get repaired/updated pretty quickly - it's the items like sidewalks that are under the purview of the government itself that "slip through the cracks".

What's the alternative that reaches eventual compliance everywhere on a shorter span than "wait for the grandfathered buildings to be bought and demolished" though?

A daily fine for everything noncompliant. Make the fine approximately proportional to the number of potential users impeded.

Then a popular crosswalk gets upgraded before a rural crosswalk that nobody uses anyway. An easy upgrade is done before a hard upgrade. Sometimes even new stuff might be built non-compliant if, for example, there are other substantial benefits that outweigh the fine.

Just burn the fine revenue. (Burning money is the inverse of printing money, helps reduce inflation, and means nobody has an incentive to maximize fines)

I have trouble imagining anyone convincing a city to 'burn revenue', even if it's a good idea in principle. Maybe the next best thing is applying it to a negative income tax so that it goes to the lowest-income residents.

It only makes sense for the federal government to mandate burning dollars.

They're the ones who can print them anyway. The end result is they just print less each year.

It's a reference to Modern Money Theory which states that a (federal) tax is disconnected from spending because the government prints money.

But ADA stuff is almost always handled on a local, not even state, level.

Impose a supplemental property tax, increasing over time, on buildings out of compliance with current requirements, so that upgrade or replacement becomes comparatively more desirable vs. doing neither than it currently is.

Or, just have a fixed window to achieve compliance or the building is condemned.