That would leave them in exactly the situation they are in now, except they won't be earning a worthwhile league bonus.
If all the TH10's drop to silver then not only would they get attacked by other 10's but if there is anything worth having in their collectors then they'll get attacked by lower TH's too, simply for that loot, regardless of whether the attacker ends up shielding the defender or not. High TH's in low leagues simply do not care about stars or bonuses, they only want loot.
If I was in silver right now, I'd be hitting any fat TH10's for loot (I'm a TH9) without caring on the final outcome other than getting the loot.
But, I'm currently in masters and the bonus pays for my army so I DO care about getting at least 1 star as well as the loot. Consequently I next past most of them.
QED leagues do matter. Not only can you earn more in bonuses, but it also changes your army comps and your style of attacking.

