You can do that, but you still have a battery that will eventually become a problem. I prefer my wired devices not having a potential bomb on board :) As I generally use old devices with worn batteries this is likely to become a problem much sooner, and much
less of an issue to stick a screwdriver in and start tearing parts out of the device. Of course you're not going to do this to your brand new tablet under warranty.
However, doing that you do have the benefit of having "UPS" protection of course (how much depends on the charge cycle). In my case this isn't really a problem because if my power is down there is not much in my home left to control that still works.
My method has some drawbacks too - it looks bad (though it can be mitigated with a 3D printed enclosure) and sometimes stability may suffer, because tablets and phones use their battery to absorb performance peaks which require high power drain. The DC-DC converter will not live up to the same peak capacity. But generally I have not really seen many issues in practice. The converters stay cool and the tablets don't crash. I think it's because the workloads as a control tablet is not super heavy either. One of the main things that really peaks a lot in phones is the mobile (4G/5G) radio when it transmits and of course I keep them in airplane mode in this usecase.