There are plenty of ways to work 1 person can be all classes, without ruining the way the game works too much, I feel like. A few examples:
Only allow one class per patch (or restrict it to every month or something) to allow gear to be equipped beyond a certain Item level. This way if you pick a Rogue, you still can't play any other class in higher content. The highest item level that heroics offer, maybe. I feel like that would be a good place.
Simply just don't let you hit level cap. Leave you 1 level away. This doesn't seem like much fun though. I feel like twink PvP would be pretty popular at almost max level though.
Once you enter a raid with a class, you're locked for the week in that class for raids. This could still end up with guilds trying to force you to play certain classes that week with the offer of gear for your other, but I feel like this might be okay. Wouldn't really want to see this though.
In any case, a change like this would be pretty popular and would give people a lot of "content" to do for their character. Especially for the hundreds if not thousands of old quests people just left barren.
Extremely boring very fast.
Sounds lame.
I'll quit some MMO's very early if I can't find the right class for myself. Imagine the only DPS being a Mage, or the only tank being a Pally. Boring!
Tera had this problem for a long time. Just one real tank class for the longest time and it made me never want to tank (my preferred role).
if you guys hate ideas like this, then you must advocate for less overusing of the 5man gametype for hardmodes and an introduction of exclusive 10man hardmode maps, because if you do want to keep a mountain of specs locked behind grinding for gear and levelling then you create brutal metas (for hard mode gametypes/rankings) that can only be alleviated(not eliminated) with 20man or anything bigger than 5.