EDIT: Apologies. Updated with a link to what gorhill REALLY said:
Manifest v2 uBO will not be automatically replaced by Manifest v3 uBOL[ight]. uBOL is too different from uBO for it to silently replace uBO – you will have to explicitly make a choice as to which extension should replace uBO according to your own prerogatives.
Ultimately whether uBOL is an acceptable alternative to uBO is up to you, it’s not a choice that will be made for you.
Will development of uBO continue? Yes, there are other browsers which are not deprecating Manifest v2, e.g. Firefox.
The vast majority of people aren’t even going to know or care. A lot of people will probably just continue on even when their adblocker becomes less effective. Of course the type of people who use adblockers are also more likely to wonder why their adblocker suddenly became less effective and then switch to a browser where it’s more effective.
It takes a lot to change the inertia that already exists. People have been predicting the rise of the Linux desktop for at least a decade now and it still hasn’t really caught on.