IMO It's not the number of teams that makes the impact, it's the number of games/repeat fixtures that has the bigger negative impact.
14 teams works because:
26 games, each team played home and away, thus removal of fabricated/slanted fixtures to have more local rivalry matches, it works better IMHO, I gave an example on the VT, 2x sell outs as opposed to 3x with just slightly more fans but more costs overall to host 3 games thus less net income, also multiple/repeat fixtures for the 'derby' games = fan fatigue hence fall off on those repeat big games..
Ditching 'magic' weekend, I've not seen evidence that this benefits clubs financially, given the cheap tickets are significantly less than a std ticlet (less than half the revenue for a 2 day ticket compared to 2 full price tickets) and the effect it has elsewhere including the home teams loss. That's before the some fans investment in travelling/boozing which does have an effect on CC games incl the final when those neutrals might have had a big day out there instead should their team not get there.
Newcastle already had roots and a team that had done ok, when I assessed attendances from pre 'magic' I can't see any influence holding magic in Newcastle has had than if just the normal progression from same input into an area at grass roots/development. You don't need to input grass roots/support to fledgling teams by holding a 2 day repeat fixture event that loses money to clubs.
Ditch the Easter Monday game, this is a player welfare issue IMO and is also fan fatigue, I don't really want to watch two teams flogging themselves when half the team are shattered, no problem with squad rotation but you can't replace everyone in the 17 and as we've seen, player fatigue leads to more injuries and can have a significant effect on the quality of the game itself.
Have your play-off at the end of season but change the format back to what we had of top team are champions. Do top 8, simple 1v8 etc as per old premiership but teams MUST field minimum 50% of players say u23/players or those with fewer than 6 1st team appearances (just a figure I plucked). If we are to have end of season internationals then player fatigue/injury plays a huge part on how the national team/s do in these late in the season games.
Do better at International level equals more exposure, more fans and so on.
Does less TV money per club from more teams playing in SL have an impact, yes, however with more teams but fewer games overall I beleive that that can be overcome through higher overall attendance averages whilst including more teams in the top division.
oh and also scrapping immediate relegation, every two years to have P&R, and more suport for teams coming up so that they get a fair shake regards preperation for the step up. London did amazingly well and not keeping them for the following season has had such a massive negative impact on the club and further the sport as a whole.
A chance to regroup for a second season to consolidate could have seen a resurgance, it might not but we'll never know and now I feel London, or any other club that comes up wil be damaged by the drop back down after getting hammered most weeks. It's so imbalanced. But London particularly because of their circumstances, location, lack of deep rooted history (40 years at the point Fulham came about misses deeper problems regards getting a new team in one of the most diversified populations and congeste social activities areas and of course the 'sport' that dominates.
10 teams is an idea, but not one I could support, I don't think it helps the sport overall longer term to survive never mind flourish.