I agree with you on Bogaerts. For all of his inconsistency, he's had multiple half-seasons where he's looked like a future batting champ, a full season where he hit .320, and another with 20+ home runs, all before age 25. If he ever puts it all together, there's a terrific all-around player there, and IMO he's still too young to write off the possibility of that happening.
Even in his somewhat disappointing 2017 season (the one where he played hurt for the whole second half), his value relative to Machado was essentially a toss-up depending on which version of WAR you're using...it's certainly likely that Machado will have a better 2018 season than Bogaerts, but I think it's less of a certainty than many probably believe, and that's before factoring in Xander's additional year of control plus whatever else you'd have to add to the trade.
Keeping the left side of our infield as-is and adding a bat at DH just makes the most sense to me...we can always pursue Machado and trade Bogaerts (or move Devers to first) next winter if we wish.