I'm a traditionalist in some ways, but ultimately the important thing is that the calls are right, not who makes them.
I like the idea of managers making challenges and there being a restrictive limit. I would have just said 2 challenges because no good manager is going to use one too early, but whatever number of challenges they'd decided on would be unpopular. It can be adjusted. It might have been wise to try all of this out at minor league level for a year or two.
It is inexcusable that MLB is 5-6 years behind Wimbledon (by far one of the most traditionally thinking organizations) and Cricket... a sport with plenty of time on hand... at least at Test match level. In the case of Wimbledon yes, gamesmanship takes place and some time is wasted but everyone accepts that the call is corrected if necessary and that is the most important thing. It's worth it.
The important thing to recognize here is that the Rubicon has been crossed and there will be no going back. That is significant.
In my mind we are not very far from umpires all having handheld devices that they can consult themselves almost instantaneously. The umpires should be leading this development because it's coming with or without them.
It is ludicrous to have situation where the entire world knows within seconds that a call is wrong but it is allowed to stand irregardless. It's fixable, so fix it.