I would generally agree in this particular egregious case, but the game has no technical obligation to make that happen.
The gacha model makes it a point that you don’t buy heroes per se, the product is the gambling process. Heroes are specifically framed to be a shiny portal ad, but not the product.
It’s exceptionally sleazy, but those chasing after a particular hero are considered pre-warned about the risk that they aren’t going to get what they want, and even if they do, they might not get what they expected from it.
Besides, balance changes are a thing even in games like league of legends, where you can buy specific characters directly, and then a patch or two later, they’re nerfed to be borderline unplayable, or reworked to have an entirely different identity and abilities. You get what you pay for at time of purchase, but there are no guarantees. That game, however, has to think about competitive integrity a lot more, with multimillion prize pool tournaments and whatnot. Here it’s not that much of a case.
But I digress. My point is, nerfs and buffs are pretty essential, and getting meaningful nerf compensation is about as likely as SGG making you pay extra to make use of buffs.