Altcoins can never be a threat for Bitcoin because Bitcoin is the king  and all other alts are following it. We all know that I never heard this before TBH that alts are threat to BTC, the reason you mentioned that other said, are also illogical to me, in short we don't have to stretch this out because reality is in fron of us.

We know alts follow BTC, and most of the dominance is of BTC too, and when people invest in alts they take profit they have to swap their profit into more stable and profitable asset and for them the easiest way is to conver them into BTC so now or in near future they will invest in BTC.