I disagree. Yes, Mastodon is the first ActivityPub-based platform for most people, but that doesn't mean it's obligated to be an app for all social media on the fediverse. Most people go there for microblogging, not to see threads from Kbin or images from Pixelfed or videos from PeerTube. They just want a Twitter alternative, and Mastodon should provide that imo. If people already view the mere act of selecting an instance as enough of a barrier to prevent them from trying it out, you're going to lose more people if you complicate things by shoving YouTube-style or Reddit-style content into the platform.
This also makes me think that it wouldn't be very effective way of supporting other platforms. You're not going to get much engagement from Mastodonians on other types of content because those types are just not what they're there for. At best, you'll get people commenting on Lemmy threads thinking that they're microblog posts and thus not actually understanding the format of the thing they're replying to.