Stadium attendence is certainly down, can't really debate that. However other outdoor activities have trouble too, people just are in their homes a lot more due to TV and the internet.
As for tv: ratings seem to be somewhat down recently however cable stations are still paying record sums to get broadcasting rights so they are not seeing that as a dying market at all.
The transition from cable to online streaming will be tricky for baseball though as the stream likely means that it will be sold separately and not so much packaged with other stuff. We will see if prices hold steady when cable dies and gets replaced 100% by streaming because if baseball has to be bought separately on an account like mlb.tv especially younger people might be more willing to leave it than old people were to leave cable which has other stuff than baseball too. Obviously it could also be on a package streaming service like disney+ or Netflix but the price pressure in that market is very high (just compare the price of your cable with Netflix or disney+).
MLB is doing great financially but they are worried about a decline of young viewers as well as some minorities which why mlb is trying to Adress that in some marketing campaigns.