STM32F103 and STM32F407 still work perfectly in most applications, with plenty of alternative chips in the market, I don't see a reason to move to new product lines.
I don't know what "most applications" mean. This kind of assertion is always odd. What is the definition of "most applications"?
STM32 MCUs have made big gaps in terms of performance and, particularly, power consumption in more recent series than the venerable F1 and F4. If low-power is a requirement, then the F1 and F4 are certainly out of the equation, so in itself is a good reason for considering more recent series.
There are alternatives for sure, but again, when it comes to low-power and the L4/L5/U5 series, not that many either. Very few actually that come even close.
I somehow would translate your above assertion to "STM32F103 and STM32F407 can be had dirt cheap in China and we don't see any reason to use any more recent STM products, as they are much more costly for us. And we don't particularly care for low power anyway."
And with that said, with anything that doesn't require ultra-low power, I now like the WCH RISC-V-based products, but their peripherals tend not to be as flexible as what we get on STM32 MCUs in general, at least the more recent ones, as WCH tends to "clone" STM32 peripherals from older generations.