"Ability to cross-post from Twitter works against Mastodon adoption"
My thesis on this is simple. People choose the easiest path. If the destination is "try Mastodon", one path is "log on and use Mastodon" and the other is "set an automated cross-poster, keep using Twitter". So, people who might have otherwise committed to using Mastodon, for there not being another option to "get on it", now have the option not to.
I have observed this trend before and after the cross-poster became available.