Is F1 really comparable to any other race series? I'd say no. Merc has spent 1.2B+ over the last few years. There is zero equivalence, nor anything to learn there.
The USA is a unique market with their own, traditional, series and very limited interest in non-american sporting events (soccer, F1, MGP don't do as well in the USA for a reason).
I see very limited appeal for MGP as an "entertainment product" without prior interest.
I totally fail to understand your point that participants from a wide range of countries, speaking a wide range languages means that MGP is NOT an international sport, unless you are solely focused on MotoGP. In the lower classes, often watched as much, if not more than MotoGP itself by my SE Asian friends, they love hearing Thai, Bahasa Malay, and Bahasa Indo. The biggest future markets use small bikes, watch small bike racing and buy Repsol branded 150s. The sport is FAR from niche here.
Fight sport is an extremely poor comparison. You seem to fail to understand that fight sports DO have international, universal appeal, racing sports do not.
Dorna should most definitely not be trying to fund global feeder series. It would be impossible. And there is so much grass-roots racing of small cc bikes in developing countries it is totally unnecessary. What is required, as you mentioned, is a path to MGP for promising stars, Reza and Idemetsu have invested in this.
One thing always overlooked when judging the competitiveness of racers from where I'm from (equatorial region) is the geographic and cultural adjustments necessary along with the adjustments every non-equitorial rider must make when moving up classes. Subjective example: Wilairot was always slow after lunch in his first year in Europe - he was used to a traditional large lunch followed by rest in the heat of the afternoon. Taking riders from non-euro climates and geographies takes additional adjustments.
Currently I am preferring the on-track show of the S1GP championship to MGP. https://www.supermotos1gp.com/
Formula 1 is the closest comparison that can be made in my opinion, and whilst historically F1 has not done well, that has definitely been changing over the last couple of years. A hit netflix show, an American team and as of this year three races in the US is showing there is interest, and more importantly investment. Not to mention the new owners, Liberty media are American and have completely revamped the promotional model of F1. The early signs are they are being quite successful in their efforts.
My point regarding languages is that Spanish and Italian being the main languages spoken by the majority of the talent shows not enough is being done to widen the sports appeal, although you do correctly mention that in some of the Eastern territories this doesn't seem to be much of a factor. Merely my observation is that a global sport would ideally feature a global cast of competitors, and currently MotoGP doesn't to the extent that it could. I'm not suggesting that I'm turned off by the fact there aren't more native English speakers, quite the opposite.
If combat sports have a wider global appeal but motorsports do not then why do you think that could be, after all motorcycles are ridden across the world? Genuinely curious here.
Also, if grass roots motorsport is so prevalent in the eastern markets, then obviously Dorna are doing something wrong, because the vast majority of talent that graduates it would seem are of Latin origin. In my mind part of the issue there could be a lack of interest from larger sponsors from that part of the world. Petronas gave it a good go, had immediate success then decided it wasn't money well spent. And yet they are/were bank rolling much of the Mercedes F1 budget, so perhaps they felt it wasn't a good investment from an exposure perspective.
My view is that the dearth of truly international talent making it through the feeder series to the top class is that big money sponsors just can't see an obvious return in sponsoring youth the way they do in F1 for example. Take Pastor Maldonado or Sergio Perez as obvious examples. They were funded all the way through the junior categories because their large national sponsors knew that if their drivers made it to the Premier class, they would get international exposure they otherwise would not. If Dorna were genuinely effective at growing the sports fan base and worldwide audience then maybe we would see more of the likes of Idemitsu and Petronas equivalents from different territories seeing value in investing in fostering talent.