...about how far I could have gotten by now had I just chosen a main and kept it. I have four characters in their 30's, one level 44, and about 5 in their 20's. I could have gotten a character to 90 by now!
And yes, I started when MoP started.
Did you have fun? I hope so!
If you only consider World of Warcraft in the context of how 'far' your main character gets, then that could be a recipe for needless frustration. Personally, I find the time I spend with my alts is well spent. I enjoy questing and doing lower level dungeons, and I find it fun to learn differing class mechanics.
With all your time spent experimenting and leveling alts, you've now got a lot of characters that you've gotten a feel for, so you're in a good position to pick one that you'll have a lot of fun playing when you get to end game. You'll also have a bit of a head start when the time comes to start leveling those characters up too.
No need for regret!