You sure about that? Look up Visible, Mint Mobile, Total Wireless, US Mobile, Tello... Same carrier networks, same quality of service. You can even pay a bit extra for prioritized data and other fancy features. You can get basic unlimited plans for $15-30 and premium plans in the $30-50 range vs $100+ at the big carriers. The only difference is that you aren't paying for your "free" phone.
In this case you’ve switched providers, though. Might be a good idea but doesn’t say anything about whether, e.g., ATT will lower your price if you bring your own phone.
I’ve read that these virtual networks also get lower prioritization so you can get low bandwidth when the higher tier users are active. Not sure how accurate that is.
Broadband Map tracks priority levels for the big three US carriers' plans and the MVNOs they support. I paid $225 promo pricing for one year of Visible+ Pro with unlimited priority Verizon data and all taxes and fees included ($18.75/mo), so I can pay full price for a flagship phone and end up ahead of any carrier phone deal.
That’s the whole point of buying your phone unlocked: to allow competition between carriers. Growing up in Europe we had very cheap prepaid plans but you still paid more for out of network than in-network, so lots of budget conscious people had two or more prepaid sims and swapped between them. No monthly bill and you add money to whichever number is low. It helped that receiving calls or sms was free, so you could ring someone and have them call you back if you were a bit low on funds.
Man thinking back, I probably got away with less than 2 dollars a month back in 2006-2010 era.
And if you don’t use a lot of data, at least US Mobile has a by the gig plan. My family has three phones on it for a total $30 per month. Those months that we go over, it automatically charges $2 for each extra GB, with data pooled between the lines.
It is easy to switch between Verizon, AT&T, and T-Mobile as well. This was helpful for me as all three of the networks normally have one bar or less at my house. T-mobile WiFi calling works more reliably than Verizon.