I see this argument that Crash and Spyro have to come together quite often, but I heavily disagree. I love both series, but it's not like they're on equal footing, it's more like a big brother and little brother relationship, with Crash absolutely being the big brother. When we're talking about PS1 classics, Crash is the very first name on a lot of people's minds, it's very easy for that series to be in the conversation on its own, where as it's rare for Spyro to be in the conversation without Crash being mentioned first. That's not to downplay Spyro's success, but Crash's legacy includes everything that Spyro's does and more, he can easily stand on his own in Smash.
Furthermore, Activision can't just run the rule over Nintendo, if Nintendo want Crash, then that's who they want, if Activision were to lay down demands like you describe, I imagine Nintendo would just scrap Crash and look elsewhere, they have no reason to be so desperate. For the record I'm pretty sure Activision would be one of the studios that would happily accept just having anything in Smash, Crash getting into Smash would be a huge business opportunity for them as it'd be a big boost in the already strong momentum the series is enjoying at the moment.