Watching the Heat game makes me think that it's not so bad if we end up losing someone like Crowder in a trade for someone like PG or Butler. There are a bunch of high end role players (or obscure bench players waiting to be tapped) out there. Like, look at James Johnson. Or Tyler Johnson. Or in our case last year, Evan Turner. Yes, Crowder is very good but if you could get a guy who brings 80% of what Crowder brings while landing an all-star, all-NBA talent like PG, it's something to think about.
Nobody thinks that Crowder's production is irreplaceable. The issue is replacing his production
at his current salary(Right now he's an above-average starter at an 8th or 9th man price). With the three guys you named, that's not possible.
James Johnson is only making $4 million this year, but I can't see him agreeing to anything short of $10-12 million/year this summer, which puts him way out of Crowder's price range. Tyler Johnson, on the other hand, will be making ~$1 million LESS than Crowder next year, which is an incredible deal. However, that balloons to ~$20 million/year in 2018-19 and 2019-20 (last year is a player option), which is insane. As for Turner, I think we all know that the 4/70 deal he signed was crazy.
Obviously you aren't trying to say that we should add those specific guys, but just that players are available with similar production to Crowder. And you're absolutely right. But the main source of value for Crowder is his insanely low long-term salary. You can't find anyone out there with similar value for the money, and any that you do find will be held on to tightly. Crowder should be as well.