The thing about William Shatner is that he has the monopoly on being William Shatner, and he can get work for the rest of his life, doing nothing but playing William Shatner.
I've seen a lot of Trek actors become character actors who appear as guest stars or recurring characters on network dramas.
Who else could get people to buy music albums they expect to be bad, solely on the virtue of having been performed by William Shatner?
I think it was casting Shatner as the captain in the 2nd pilot that sold Trek as a viable property. No real evidence, just my gut.
He has that smirk and twinkle in the eye that pulls people in. No other Trek actor was as successful before, and none since. Not even Nimoy. Heck, he was cast as the lead in a series at 70+ years old. Twice. Not to mention the multi-decade ad campaign for Priceline.
Who does that? Shatner, that's who.