I mean, I guess it’s a big deal for people who can’t be bothered to look out for D.Va using her Defense Matrix and exploit her lengthy downtime windows every time she uses it. Just like the people who can’t be bothered to NOT shoot a bubbled Zarya or her bubbled teammates in order to defeat her more easily. Same with shooting a deflecting Genji.
That was a long, long time ago, and I played back then too. Defense Matrix as an ability didn’t last long in the live game, but I remember the RMB “fuel gauge” design she has now did exist before the 1-hero limit was implemented.
For reference, from this thread
And the single-hero limit didn’t come until later, as this Reddit post from November 4, 2016 discussed:
Single-Hero-Limit will come to Quick Play!Don’t panic! There is a game mode in Arcade which is called “No Limits”!
As for your other comment about 15 m DM:
In practical play, any decent D.Va will be very rarely swinging her DM around madly in circles or very wide arcs. Her DM is forward projecting, yes, but it is almost never covering a circle with a 30 m diameter. A D.Va that does this is not being focused on what is going on around her while also wasting precious DM uptime—which again, also creates an exploitable downtime window—and all of which makes her and her team vulnerable to counterattacks and ultimates for doing so.