Quote from Xeruh »
Why would Bolas be unable to use magic? Losing his spark doesn't take away magic. It just takes away planeswalking. Doesn't mean he might not have some other sort of tricks up his sleeve, it's Bolas.
The Spark granted additional powers , like an upgrade to all the abilities the wielder already had.
However, that was the case for the Old Walkers, the new ones its really just barely a drivers license to planeswalk (and with stuff like the Planar Portal / Gate its not the only means to travel the planes, given you have enough technological know-how).
Keeping Bolas "alive" is for sure a guarantee to be backstabed at any moment, its not like he will die of old age any time soon.
However, such a tremendous defeat is really really bad for the psychological sanity of a extremely selfish narcissist like Bolas , so chances are if he ever gets the chance for revenge, he wont mangle around for a greater plan anymore, but seek for all out annihilation.
Keeping Bolas around however as a potential source of help even, is still a possible move ; so in case something even more crazy powerful pops up, they can join forces (which is silly, but a very common trope to increase the epicness of a power creep increasing beyond limits).
At this point the threats in Bolas and the Eldrazi pretty much tickled the upper limits of what is even remotely thinkable, as any other more powerful enemy would be quite outlandish (still have Phyrexia hanging around, but in the end, they are in general a much lesser threat than Bolas or the Eldrazi as they quite slowly work their way from plane to plane over time).