If their Youtube “quit” is for them going to college or using their money in lucrative investments, so they can achieve their dreams... there’s nothing to worry about.
but if they are trying to “quit” thinking they can keep the same following in new platforms I think that’s not a smart move. All the merchs they sell is consequence of years Vine/YT work.
If YT doesn’t give them the same earnings, it’s time to use their fame to push “less risky” options.
Grayson could easily become a full-time fashion model (Cameron Dallas did it) or they both can even try little acting gigs (just look at Netflix and Amazon catalogue, there’s plenty of opportunities to use non-actors to clout viewers)...