To answer them in order:
1) It's certainly possible for guys to turn from a contact oriented approach to a strikeout/power oriented one, but the stuff has to be there, and the pitcher has to be willing to change his philosophy. It's pretty much all down to pitch usage: which pitches to use - generally fastball over sinker paired with a good breaking ball - and which locations to put them in - high/low for power pitchers; often bottom of the zone, side to side for contact ones. For examples of this just look at pretty much every pitcher on the Astros, with Gerritt Cole being the clearest example of a guy changing philosophies from how the Pirates approach pitching to how the Astros do.
2) Technically, becoming a strikeout pitcher doesn't necessarily mean you'll be less able to go deeper into games, I don't believe there's much of a correlation between strikeouts and pitches thrown in games - keep in mind contact oriented guys may allow more balls in play becoming hits, or many foul balls. Some of the top starters in the game - Scherzer, deGrom, Gerritt Cole, Verlander, Walker Buehler, prime Kershaw, Chris Sale - are all huge strikeout pitchers and routinely go deep into games. Guys like Stroman and Mike Soroka of the Braves are rare in being both top starters and guys that can go deeper into games, but that's because their stuff is also just that good. Until strikeouts for starters become a problem with regards to endurance, that's not going to be a problem that the MLB has. Their biggest problem is with the lack of balls in play due to elevated walk and strikeout rates.
3) While I can see bullpen games becoming far more prevalent in the future, it'll be a very long time before the traditional starter goes away. There's great value in having a good starter that goes 5-6 innings, and those guys are still being developed and coming up each year. An elite starter their third or fourth time through the order is still about as good as the reliever that would be replacing them, so you're getting more innings out of less roster spots. That's not gonna change unless injuries continue to zap us of promising arms and elite starters just simply don't exist anymore, and I just can't see that happening any time soon.