I know it's only an anecdote, but myself (and many other colleagues) in Boston switched to periodically using rideshares to commute due to the decline in public transit quality. The tech boom in the area completely overloaded the system, and when you combine it with the rampant corruption behind the MBTA, this was inevitable. Uber and Lyft have not been the best actors, but someone was going to step in and profit off…
I've been riding the T in Boston for the better part of 20 years now. My usage of rideshares is more related to convenience and timing than any change in MBTA service. A lot of trips I do require going into/out of the core city to make a lateral move, or to coordinate multiple modes with different headways, e.g. 5 minutes for subway, 10 minutes for one bus, 25 for the next. The result is that time in motion is similar to the rideshare, but I spend an equal amount of time waiting for a connection.
I'd really be curious to see the effect on trips that already have a direct or reasonably efficient multi-step itinerary, e.g. Davis to Charles/MGH, Sullivan to Harvard, etc.