It can be a serious issue, yes. To use my example earlier of needing to still use my ilvl 50 AF body even at ilvl 270 content, this means that LITERALLY every healer body piece that has come out since ARR's launch is literally worthless. And that's assuming the other healers have similarly potent bonuses on their low level gear. If not, then no matter how well balanced the classes are, WHM becomes the required healer for all content simply due to having better gear. Or on the contrary, WHM is considered useless unless you have the gear because it's required to make up a deficit.
Granted, that's a bit on the extreme ends of things, but it's not quite that far-fetched either. Ability bonuses frequently wind up being a multiplier to an ability's potency, either directly or indirectly via reduced cooldowns and such. Bonuses to your bread and butter skills in particular may be prone to being too useful to replace: You'd just increase your stats on your other gear and leave that piece on forever, unless some other piece in the same slot comes out that grants the same bonus... and I think the people that ask for these kinds of bonuses aren't asking for literally every chest piece to give the same bonus. Then it just becomes a job trait.
These kind of ability bonuses worked in older MMOs because they had a different system and more importantly a different playerbase. The players who didn't bat an eye at needing to do every single piece of endgame ever released at awful drop rates (which, I will add, is kinda necessary for side-grade loot systems) are all grown up now and have jobs and various adult obligations, maybe even kids. The generation that's grown up since then generally tends to lack the free time with today's game schedules... I mean have you SEEN how many great games come out each month? And for those who do, games like FFXI and Everquest still exist, and even have legacy servers for those who want to experience the true oldschool grind.