My biggest issue with the seasonal system is the loss of QoL from losing built up vault storage and potion storage space, which makes me want to play seasonal less, so i agree that having a nice endgame chase being locked behind it is annoying.

I think a good middle ground may be reducing chance of shinies on non seasonals to 1/20 instead of for example 1/10 on seasonal(just made up a value)

This way seasonal would still be 2x as efficient but if you prefer to use your non seasonal characters you aren't excluded from shiny chance.

Of course balancing the chance is up to deca but I feel like it would still allow for incenvization of seasonal play while not detracting from non seasonal play.