Generally if something is fast enough/efficient enough that a paying customer can use it without having to worry or actively think about performance and un-bloatedness, that's enough for them. The only people who might complain still are developers who are bothered by the inefficiency and are technically literate enough to notice it, and maybe the users with less powerful/capable devices than the ones the big paying customers use. Generally these groups of people are not the actual customers of these products.

The people who actually pay for slack and discord (eg enterprises that need workplace chat app and decided to go with the "gold standard", consumers with discord servers and such) need the features/tradeoffs choosing featuers over efficiency causing that bloat. They just don't all need the exact same set of those features as the other customers. So because customers are willing to pay for all these features the product tries to ship all of them and becomes bloated.

> Every person I talked to that uses Excel hate how slow it is

But do they make the purchasing decisions behind using Excel?

To be clear I am not really arguing that bloat/overly enterprisey products are good. What I mean that you don't see the world exploding with more elegant products now with agents for the same reason you didn't see the world exploding with them before agents either: the people who pay for those products and build them for a living are not incentivized or necessarily even rewarded for choosing to make them more efficient or elegant when there are other things that customers are asking for with more $$$ behind them.