Ability to align with the runway on final using a long slip tells you little about remaining control effectiveness in the flare in my opinion. For one, as several have noted, surface winds tend to be lower. Secondly, speed on final is generally going to be significantly higher than in the roundout, making it easier, not more difficult to align with the runway due to increasing control effectiveness.
Experience plays a large role in knowing the aircraft's and the pilot's limitations in crosswinds. But angling the cabin for a mile down the final in some attempt to see "can I do it" doesn't teach much IMHO. Maybe as a cross-coordination lesson for a pattern or two to get the muscle memory for what has to happen in the roundout, but after then, not so much.
Yes, surface winds can be lower, but sometimes they can be higher. I've flown at airports where the winds "tunnel" through hangar rows and can actually be stronger. To me, that's not a reason to use wing-low vs crab, it's mostly irrelevant. I don't primarily have students slip earlier because of control effectiveness, but it can inform.
Regarding your "secondly" - partially true, but it's easier to transition from keeping it straight on final, to adding in a little more pressure in one direction, than it is to do a much larger, wilder kick, to try to align the aircraft at the last minute, especially if things are gusty.
I don't teach, am not good at it anyhow, and after 30 years, I pretty much crab down final smoking the (proverbial) Ernest Gann cigarette until it's time to round out, align, and roll on the upwind wheel. This works for tricycle and for tailwheel aircraft. The key is knowing how to use aileron and rudder (and possibly power in miserable winds) continuously to keep the nose aligned. The rest is window dressing and personal choice.
You said you don't teach earlier in the thread. You are sure offering pretty strong opinions for everyone to see for that admission. Not that that's morally wrong, but it doesn't seem like you have tried your advice with multiple students. That said, where you said this:
The key is knowing how to use aileron and rudder (and possibly power in miserable winds) continuously to keep the nose aligned
This is actually the reason to teach students to slip earlier on final. It provides them with a LOT more time to practice keeping the nose aligned and the aircraft on the centerline, and that helps them more than crabbing for a mile and then spending 10 seconds and 200 yards or whatever it is trying to learn how to keep the nose aligned.
My sample size on students says that it indeed is easier for a tailwheel rookie to transition from the wing low slight slip on final to the sight picture in the roundout / flare phase.
This is in the "pilot training" forum, so IMO, it's better for rookies to try the wing low. Again, once you are proficient, crab all you want.