This is why we can't have nice things.Very much a known problem. Unfortunately, there's not a good way to fix it, particularly the way that boarding order/priority is currently incentivized on airlines - whether that is for overhead bin space or for seat selection. You can't make people disclose their disabilities and you can't put up barriers to entry for people who legitimately need the assistance. So, the system is ripe for exactly this abuse.
The solution is to pay for the seat you want, and either a) have high enough status to board as early as possible, b) only carry on items that will fit beneath the seat, and/or, c) be amenable to your carry-on luggage being gate-checked. There is absolutely no sense in stressing out about the jerks who abuse the system.