Really not a fan of season 1 to start because of this. I said they needed to tune down how fast these ults charge, While i didn't think people would just realize you could chain them to become unkillable, i feel stupid for not putting that together sooner.
Ran into a comp running Luna/Mantis/C&D and they had an ult locked and ready to answer anything we did. It was obnoxious. No skill required to really run this either because of how easy it is to build ults. In a regular game i can usually build Mantis ult 2 times in 4 minutes. If my aim is hot or the tanks give me free shots at them enough, it'll be 3 easily.
It is kinda strange it took the community this long to figure it out, but i suppose shit like GOATs didn't pop up overnight either.
I feel like the nerfs to Hawkeye and Hela allowed this meta to take hold. The previous meta in Season 0 was mostly about tanks sponging up damage, while high-damage ranged heroes dealt damage from behind. Plus, a healer like Mantis to keep people alive.
With Hawkeye's effective range decreased and Hela made squishier, and no real substantial buffs to any DPS characters aside from Storm (who is meta now) and Bucky (who has kinda replaced Hawkeye as the most effective burst damage DPS on a lot of maps), good players realized prioritizing survival over damage was the better play. The hockey equivalent is a team deciding an iron-clad PK is worth more than a white-hot PP... which, ironically enough, also proved to be the winning strategy in playoff hockey.
There are a few options the developers can take.
1) Make DPS character ults more damaging. If a DPS character ult can reliably kill players through a support ult, it disincentivizes piling up support characters and stringing ults together. Ideally, this would push players to use their support ults more strategically, to use their ults away from the heat of the battle in preparation, or to regroup, rather than just chaining 'lol, you can't kill anybody' ults over and over.
2) Increase how long it takes support characters to build an ult. This was your suggestion, and it could work. Still, I feel like the unintended consequence is that we'd get the inverse of the current problem, where supports are rarely or never chosen because the math suggests going 'all damage, no healing' is the better payoff. While that might make for more exciting games, I'm not in favor of essentially rendering an entire class obsolete.
3) Improve Tanks. This is my personal favorite option, cause right now I feel like playing as a Vanguard is largely pointless and deeply boring. What is the point of being able to tank damage, when you can just as soon heal everybody's damage just as fast as a strategist? Strange and Magneto are meta largely due to their utility, not because of their tankiness. Penni Parker is useless outside of defense in specific modes, Captain America might be the worst hero in the game alongside Black Widow, Groot is decent, again for his wall utility, not cause of his tankiness. I think more Vanguards should be like Thor and to a lesser extent, post-patch Venom, in that he's actually useful in the fight to deliver damage as well as absorb it. Suppose your team composition includes players that can deal damage and stay alive without healing ults. In that case, you're more incentivized to pick all sorts of characters, rather than relying on constant healing to keep squishier DPS and low-damage healer characters alive and in the fight.
4) Extend the seasonal bonus/buff beyond specific teammups to entire classes. This isn't particularly likely cause it's a 'bullet point on the box' type feature, but it's ironically the easiest to implement and will immediately have the biggest impact. Give different classes or team comps different bonuses. The more healers you have on your team, the less healing each individual healer does. The more DPS players you have on your team, the longer their ults take to build from damage dealt. The more tanks you have on your team, the lower their HP bonuses get, etc. You could tune this season by season based on how the meta develops, and it wouldn't require major character tuning (although they'd probably need to do that too as players find exploits and whatnot)
