Do you think Game Freak will continue to add new evolutions to existing Pokemon now that mega evolutions exist?

I think the two are largely unconnected.  Think about it: generation V had no new evolutions of existing Pokémon, before Mega Evolution was introduced, and generation VII featured neither.  In fact, the only new evolution of the past three generations – Sylveon – actually coincided with the introduction of Mega Evolution.  It’s true that both serve a similar thematic purpose; they both give a new generation of games a more concrete link to Pokémon’s past.  I agree that new evolutions seem to be on the way out, but I think the “replacement” concept to fill that role, if any, is much more likely to be the continuation of regional variant Pokémon.  Mega Evolution is generation VI’s mechanic and tied to the history of Kalos and Hoenn, just as Z-moves are generation VII’s mechanic and tied to the history of Alola, just as the Dream World was generation V’s and has not returned (though the hidden abilities it unlocked remain).  The concept of regional variation, by contrast, doesn’t carry mechanical or worldbuilding baggage, and innately lends itself to being reused again and again through new forms that express the personality of each new region.  Even that, though, is not certain; the designers may have liked regional variation as a feature of Alola specifically, expressing the unusual paths that evolution can take on archipelagos in the real world.  To me it’s most plausible that generation VIII will feature no new evolutions of existing Pokémon, no new Mega Evolutions, and no new species-specific Z-moves (I’m 50/50 on new regional variations), instead spotlighting some other entirely new mechanic that will be tied to the history of the new region and the plot of the new games.

All of this is, of course, as likely as not to be proven completely wrong within the next couple of weeks (heck, maybe even days).  I don’t do predictions; it makes me ill-tempered.