To me it's more the issue of creating new holes to fill. Nobody really knows what Small or Mitchell would be; they might be stars, they might never make it, they might become role players. You never do know with any one individual prospect, but you have some sort of "average" or "most likely" outcome for them that you have to count on, and so you get a certain production from the farm as a whole even if individuals will exceed or fall short of that. Now, all this is just to preface the usual retort that they're just prospects etc; the point is that while no prospect is ever a certain thing, that's not an argument to just trade everyone. Brewers will need a LoCain replacement soon, and there is always a need for SPs. Look at how hard it is to find a good CF, look at what even mediocre starters command on the open market.

So trading away two of your best prospects like that takes away two potential options to fill those holes for free; leaving the team to scramble to fill those spots down the line the same way you now try to fill 3B. So doing such a trade will need to be for a player who is truly worth that, and/or for a trade that brings you past a certain treshold or is the last piece of a championship team.

Which brings me to the most important point: Candelario is neither. Maybe he found something in 2020 or maybe he didn't, and it's all BABIP-driven small-sample noise. Either way, he's a 27 year old RHH 3B with a 95 wRC+ with three years of team control remaining. The Brewers already have a 27 year old RHH 3B with three years of (cheaper) team control remaining, who has put up a 97 wRC+ for his career so far.

If I had to guess at who will have the better 2021, then I'd lean towards Candelario. But enough of a difference to give up two of our best prospects and add a couple of million in payroll? Not even close. Keep Small (who might even see the majors already in 2021) and Mitchell for the future, and spend that $2m where it can best help. If we had a system like the Padres did, then I'd be a lot more positive towards trades like this. When Mitchell would be our 3rd or 4th best OF prospect, and Small just one among 10 promising arms at AA-AAA level. But we don't. And with how good the Dodgers and Padres and to a lesser extent the Mets and Braves look, this is not a year to go all in. Make incremental improvements, improvements that don't hurt the future, and do what you can to win the division and hope to get some of that Devil Magic in the playoffs.

TL;DR version: Candelario is not a good enough player to warrant that kind of trade.