If - huge if - the Phillies did deal Eflin obviously they would have to replace his WAR. Maybe that mostly comes from the two guys (Arrieta and Pivetta) who barely won any games in June. Maybe from a trade. But having Machado at SS and Kingery/Crawford (or even Franco/Crawford) at 3B is obviously a better team, and Machado's bat also buys more leeway for the RFs.
And even if you do make an apples to apples comparison of Machado and Eflin, discounting the defense, it's more likely that Machado finishes the year with 6.0 WAR total than Eflin finishes the year with 5.0 WAR total. Machado's gonna play every day and keep doing what he's doing. Eflin will probably have some bad games or a down month.
Anyway, I'd put a lot more stock in the front office's knowledge of Machado, his agent, and the league then in some theory that if the Phillies don't get to the World Series he'll want to leave. How does that work exactly? He comes here, helps lead them to the NL East title, likes his teammates, likes his manager, loves the front office guys who originally drafted and developed him, and then, what? They lose in the NLDS and the Brewers play the Astros or the Dodgers play the Yankees in the WS so he'll leave for one of those teams (few of whom will even try to get him)? The risk of him staying with whoever gets him seems much higher than that scenario. That doesn't change the fact that the Phillies are only getting him as a rental now, but that's true of every team.
I probably would trade Crawford, if the front office was extremely confident about keeping Machado. But I don't think I would trade him for Machado, unless the rest of the package was negligible. He's more valuable than Eflin IMO, just based on the overall history, even if Eflin is better (and healthier) player this minute.