This topic comes up from time to time and it's one of those where some of us will have to agree to disagree.
I spot-clay with every wash, VERY gently. It does NOT seem to remove an appreciable amount of whatever protection the vehicle has on it. It DOES sometimes affect the beading a little but that effect doesn't seem to last :nixweiss The next rain/wash, everything is beading/sheeting uniformly.
There's just NO way, IMO, that it could have removed my SG without my noticing it. The areas I've clayed MANY times still look/bead/sheet JUST like the areas that I haven't; I can't even tell where I've done it. And I haven't redone the SG on the WRX since I don't know when (maybe last summer!).
If you have some contaminants (like bugs) that have really eaten into your protection (or are UNDER it), then the amount/degree of claying required to remove them may very well remove your protection. It's a matter of how aggressively you clay and what the individual situations are.
Even when a sealant stops beading (starts sheeting, etc.) you should be able to tell if it's still there. The paint just "looks and feels different" when it is/isn't protected. I can get nearly a year out of a good (many layers of SG) Klasse job, gently spot claying with each wash. When it needs redone (as my WRX sorta does now), it just LOOKS like it needs redone. Sorta hard to verbalize, but it doesn't look as good as it "should". It also feels different. And the QD I like to apply after a wash no longer comes off as smoothly as usual (got some steaks last time).