The "numbers" are called Discriminators and served a variety of purposes:

• Identity wise it meant multiple people could have the same username text. If you wanted to be John, you could be John#6754 and someone else could be John#1298 and both of you could be John! Now there is only one john.
• It provided parity. EVERYONE had it, therefore no one is better or worse than other excluding particular number combinations. If you were John#5363 and hated the discriminator, well everyone else had one, versus someone behind john, and then someone having to be john_87 because there's already a john

You argue that being able to use effectively the same username everywhere is a good thing. The unfortunate reality is the rollout Discord used alongside the limited number of permutations (combinations?) of short usernames makes this impractical. For example, a friend largely goes by a 4-char username, and the switch by Discord means they can't use that 4-char username on Discord anymore. It's easy to say like "well, just add something to the end", but that is exactly what discriminators did.

At the end of the day the benefits weren't as compelling as the losses (it would suck to have one's identity impersonated or username stolen, or now most folks with short usernames have to stop friend requests cause they are getting spammed with them, or the fact these accounts are valuable and can be sold).

It is understandable that some people don't really care about the matter and that's fine, but it doesn't make the frustrations other few less important.