You can run a lot of things in current MAME on a Pi 4.
The problem is mostly RetroArch / LibRetro's making. They offer various old MAME builds that everybody has stuck to, rather than people still doing native ports, or hand picked-ones closer to what each specific piece of hardware can run.
Aside from 'current' (which nobody using RA seems to bother with as it's an always changing version) the newest 'named' version anybody uses is from over 10 years ago, and a butchered one at that.
As a result, rather than using something suitable, or a mix of suitable builds, people end up using 2002/2003 builds 'because everybody else is' It's a sad state of affairs really, and a major regression from times before RA existed, when people were starting to do native ports of the most suitable versions for each platform. RA presented an 'easy option' and the masses took it rather than trying to do things properly anymore.
Even on a PC RA really messes up some MAME features, and inexplicably kills performance in certain drivers.
MAME can't fix what is basically a problem that's now part of the culture / scene surrounding those things.
In terms of MAME's requirements we will continue to move forward, within the envelope of what current PC hardware allows. Some other projects have been held back by about 15 years due to trying to keep within the performance envelope of those devices, but when good emulation (eg the discrete sound sims) really can't be done properly without high requirements MAME isn't going to ignore the advances in hardware that make things possible; we'd rather be giving people reasons to use MAME over something like a MiSter than looking at the trashcan end of the market, and the discrete audio stuff is an area MAME can offer something the MiSTer struggles with.
There will always be things in MAME that are actively improving that are going to be out of reach of whatever the latest Pi is, because the simple fact is a $30 machine is always going to be lagging significantly behind the curve. As I mentioned tho, plenty does work, not every improvement shoots the CPU requirements up significantly (eg. most of the proper protection emulations are fairly lightweight) and people using old builds for games where newer ones would work better is a scene problem (always trying to accommodate the lowest common denominator) not a MAME problem.