Post merger I went from a $25/month t-mobile plan to a $15/month t-mobile plan. Not entirely apples to apples, but close. My spouse went from $30/month to $25/month on Verizon for more data. And there's lots of options at https://prepaidcompare.net/
The problem with MVNOs, at least that I'm noticing as a Google Fi subscriber, is I believe MVNOs are deprioritized. In a congested area, folks who have direct service with Verizon or T-Mobile seem to have better reception and bandwidth than I do.
That being said, most ARE and they are damn near unusable now in places that are growing.
Here in Orlando, Mint mobile can't even stream Spotify if you are stuck in traffic, that's how bad it has gotten. Forget being downtown or at an event of any kind.
I use Google fi specifically because its not lower priority.