Well Renault and Mercedes both make small Euro-boxes with small engines, they want F1 to adopt the new regulations (Renault were particularly keen on the proposed 4-Cylinder in-line that Ferrari was dead set against) not for some idealistic rubbish about encouraging more manufacturers but because it would do their sales good. Mercedes, Renault and Ferrari supply the maximum number of teams that the (current) regulations permit. Enough to make building their own F1 engines viable, possibly even profitable.
I doubt that any manufacturer would be tempted to join-in on the basis that the engine configuration suited their market profile. They've seen the way that the three latest teams have struggled, the risk that they'd fail spectacularly would be too great to chance. If VW/Audi, who have quite a bit of racing experience, won't take the plunge, Porsche won't come back, Honda, who have a very long association both as an engine supplier and race-team and Toyota haemorrhaged vast amounts of money to achieve sod-all, why would Kia or Proton or Tata try?
Ferrari don't build "relevant" cars. They'd be happy to stick with V8s or even a return to V12s I suspect. Ferrari have always only ever been interested in dominating F1, even if that means less than a handful of teams competing (more chances for them to win).
No. Renault and Mercedes-Benz are using the "relevant" argument fallaciously. Whereas Ferrari are at least honest that teeny-weeny engines are not relevant to their road-car business.
I also question whether more manufacturers are needed. The grids are more full than they've ever been, there doesn't seem to be any rumblings from teams about to withdraw and from an infra-structure point of view, many circuits couldn't accommodate more teams in the pit-lane anyway.
There's still a split within FOTA along the old FOCA and FOMA lines. The Manufacturer element still believe that a FOTA stacked with big-money manufacturer-teams would have more clout. I doubt whether any of the 'Garagista' teams want to see changes as they'd be the ones who will have to pay whatever the engine manufacturer's demand (and one thing is certain, teeny-weeny engines won't be supplied for any less cash).