ok...and? she also has more then 600 hp. do you think martials should be able to get more hp then tiamat because that hp is only meant to last a single encounter? she's literally a goddess. she's going up against an entire party of level 20s, and it's more then possible that all of them are casters - she's going to be focused far harder then most martials usually will (especially considering she's likely the only enemy piece on the board for most groups fighting her).
also, tiamat's saves are ridiculous. if you use the aspect, her lowest save is a +5...in intelligence. yeah, good luck feebleminding tiamat through 5 legendary resistances when every single other one of her saves has 50/50 odds or better of passing. if you use the other tiamat statblock, her lowest save is still int, but it shoots up to a +8, so now even feebleminding with her resistances gone is a coin flip. the only pcs even approaching having saves this good are those in an aura of protection (i mean...unless your players were psycho enough to roll up a high-charisma-paladin only party and they never stray more then 30 feet away from each other if they can avoid it, in which case...uh...good luck with that, you're gonna need it).
also also, keep in mind that very few groups actually fill out the adventuring day. most groups only have probably 1-4 encounters a day (i know both my groups fall into this category, and one of them isn't even 5e!). the "adventuring day" is a flawed concept to begin with.
oh, no, i absolutely do, i just don't think giving martials 4-7 legendary resistances actually addresses that gap in a meaningful way.
that said though, i think the save system in 5e is stupid anyway, and i think legendary resistances are a response to how stupid it is, so...i guess there's that.
yeah like i said the adventuring day is a flawed concept.