Why do some people consider SLP a giant wasteful attempt at on-chain tokens? From feedback I get, it's the one thing that gets the attention of people working with other cryptos

Is it a perfect solution? No. But there are far more people that are impressed on-chain tokens can functionally exist on BCH, especially considering there's no equivalent capability in BTC.

Is it as powerful as EVM-based tokens? No. But it's a lot cheaper to execute and end users have way more control over each transaction made.

Is it useful to introduce BCH to people in other crypto? Yes, especially considering how smartBCH was created to address the limited capabilities of SLPv1.

Point is, IMO, SLP tokens have done more to bring attention and adoption (particularly from ETH) to the BCH space than anything else in the past few years. Let's not "throw the baby out with the bathwater".