Can you be over-invested in an index fund?

My S&S ISA is currently about 80% Vanguard Global All Cap (original, I know) and 20% Baillie Gifford American. Baillie Gifford of course tanked in '22 and is just getting up near to break-even (not including inflationary devaluation). I'm planning on selling all the BG once it hits +0% but would likely put it all in to the Global All Cap, leaving me fully invested in one fund. This seems foolhardy on the face of it... but when I analyse it, only a real market hitting event could smash the all cap and any such event would hit any diversifier funds too, right? Also worth mentioning the sums discussed total circa £32,000 so not huge money.

Just want to check my logic is correct here.