I believe that these "problems" sort themselves out, through injury situations. This is why some of these exercises are fun but pointless at this point in time.
To fully answer your question, I would throw it right back at you and ask "do you really see 5-6 WRs active on gameday (including Slater)?" It's the same exact principle. Malcolm Butler is a feel-good, "hustle" story at the moment, and will likely be rewarded with a spot on the roster for his hard work and good showing. I'm not sure why so many are making these bold assumptions after that? Let's start with him getting a ST spot first and possibly being active...maybe seeing some real snaps in garbage time, or in situations where injuries come into play. We can follow it up from there.
While I think it's a great story, I have no idea why anyone would be penciling him in ahead of established veteran players at the moment? That said, I would expect him to move up on the chain as injuries and some possible slacking occur, so I do expect him to be active at times; and that's IF Belichick doesn't see ST value from him already, which would make him active no matter what.
At the moment, it's a good problem to have. Unfortunately, injuries can derail plans, which is often the case in football. Even if Butler isn't active for most of the games, he'll get his feet wet at times this season and likely see live game reps. If BB values him on ST and/or injuries occur, that could quickly change and he will be first in line to step up off the bench.
I think we need to see a bit more from how things shake out first, to be honest.