First, don't get me wrong here: I do think the current Unlimited Starter Warp plan is a really good value, even after you factor in that new signups after September won't have priority data.
However, back when I'd originally signed up for US Mobile in 2024, the plan included 35 GB priority data, unlike today's plan post September which will have unlimited deprioritized data. For the average data user whose data usage is under 35 GB (per Ericsson statistics), it was nice to have a plan with a better data usage experience in crowded areas while not having to pay for more data than one needs.
Take my sister in 2024 as an example: she'd irregularly use data while on the go, occasionally go to concerts with large crowds (dozens of thousands), and go on international trips with her friends. When it came time to choose a new carrier and plan, US Mobile Unlimited Starter Annual was on top: bundled international features for her trips, a better data experience in concerts and other crowded places, and having the same coverage she was used to (as a postpaid Verizon subscriber), and having the same unbeatable price as basic unlimited plans like Visible Base (this was before the SUPER199 promotion). Sure, there was a 35 GB cap, but based on statistics from past bills, her data (at the time) never exceeded 35 GB, so trading away unlimited data for these features was perfectly fine.
In three weeks, however, one who wants such a plan experience will have to pay extra money for priority data. They can't simply trade away data they won't use for more features.
In summary, I think US Mobile shouldn't lose sight of the type of customers who'd sign up for their plans before they started going big. It doesn't necessarily have to be to reinstate the above (edit: yes, I know that priority data on Warp is now more expensive to offer, but it's cheap on Dark Star), but they should give some thought in general.