This. It's become more apparent over the last few years with teams, including us, giving up the majority of their stadium to accommodate them. The current set up is nice and cosy for any team in the Premiership, it's all very similar to the conditions that led to the SPL breakaway.
The problem for the old firm now though as I see it, is that Celtic have exponentially outgrown Sevco in financial terms, since they morphed into their new entity. They've had the run of it for so long now it is hard to see how that could possibly swing back to any sort of parity even if the other half were to win a league at some point, which I personally just don't see happening anytime soon. Rangers just aren't Celtic's main focus for income these days, they'd most likely do whatever they could to widen the gap given the chance.
European football / The Champions League is only going to expand over time and Celtic will be bursting to be part of that, more so than getting 4 league derby games, so a domestic league with fewer games might actually suit them in the longer term at some point. It's only at that point I could see any real expansion of the Premiership.
16 or 18 team top tier, let Celtic crack on and do what they like with regards to expanded European involvement and focus, they'll continue to make much more than the rest in any scenario anyway, and the rest of us hopefully get a more enjoyable and accessible top league set up for fans out of it. The other up-shot being that the h*ns would likely wither again without their quarterly dose of hatred fuelled affirmation to keep them going.
Unlikely as that may be because in reality the other premiership clubs also still want the 4 OF gates a year along with minimal relegation risk. It will only happen if/when it suits Celtic.