As you say, "there won't be" as Oxford are in there twice & Birmingham have gone, even IF Northampton come to the tapes in 2026, it is still only 15, which gives 28 league meetings + any Cup or "BSN" series matches.
I can't be bothered to look at how many riders rode in both Prem & Champ in 2025, but I'd guess +/-40% or so? The only way you are gong to fill the "empty" places is with NDL riders and most of the top riders in the NDL are already in the Champ and some are even in the Prem!! Which leaves the second strings and reserves, most of whom aren't ready for Champ racing. One League might seem the "obvious way to go", but the "knock-on effect" is far more reaching as far as the overall product is concerned IMO.
EDIT: A quick look at the final 1-7's in the Prem, I make it 27 riders who doubled up in the 49 team places (hope I've got that right!!).