I would suggest that it’s 100% Atlassian’s fault that their product is so frequently poorly set up. They’ve basically abandoned their responsibility to design a good product in the name of customizability.
But when people blame Jira itself, I tend to roll my eyes, as it’s a completely self solvable problem.
I'm sure web apps don't actually have to perform like shit, but it seems like they always do.
edit: That isn't to say that Jira and Confluence are the only offenders. They aren't even the worst. Slack/Teams/Rocket.Chat are all just as bad and make me consider switching to a career without computers on a regular basis.
So if it’s not them…
But I doubt it. I run engineering at a b2b SaaS and specifically dedicating resources the way you're describing is not something I'd consider at the level Atlassian is at, especially considering there are people like you lurking out there.
Cloud Jira is secure and performant now, but some companies refuse to use it for made up security reasons, which is just one more item on the list of self inflicted wounds that make Jira suck.