The Canon EOS 80D is solid, but mirrorless in 2026 isn't a luxury anymore - it's the standard. The main advantage really is autofocus, which is incomparably better on mirrorless cameras. Day to day, you mainly notice it in focusing speed, especially if you film or shoot sports, kids, anything that moves. Battery drains faster (that's true), but nothing you can't solve with an extra battery in your backpack.
As for lenses, yeah, it's an investment. But think about it this way: if you switch to a decent mirrorless (like Canon R5, R6, or even the more affordable R50), your Canon lenses still work with an adapter. You can make the transition slowly - you don't need to change everything in the first month. Tons of people do exactly this and then gradually buy native lenses as they need them.
The real difference day to day? Live view in the viewfinder (you see exactly what's going to come out in the photo), and the autofocus is just chef's kiss. If you shoot a lot in difficult conditions (low light, backlighting), the difference is stark. Now, if you're happy with the 80D and shoot casually, like landscapes, it's not life or death. But if you think you'll keep it for 5-10 years, mirrorless is the safe bet.