this looks like a good point, but is not: it has a big big flaw. Money!
Hiunday and M-Sport doesn't commit to top level (and skoda, and lancia too btw) because of a simple reason: money! They need a return of the investment they would do to bring a new car to the top level. it's easy to see that the only working business model is selling car to the customers, even for toyota. so car is not the point, they'll do a new car if it's worth it, and it's not at the moment. u can see it also with the lack of real "tuners" for 27, we still didn't see any of them on the road, and the couple rumored working on it have big institutional backing. The reason is not the current cars, nor the lack of competition per se. it's the whole sport that doesn't generate enough income for the manufacturers. that's the point for the promoter, and that's why basically everyone is waiting.
Little bit on the cars: i don't think i20 or fiestas are that much behind. both have good results in erc and regional rallies. i20 won 2 erc in the last years, won in spain and portugal with sordo, is in top10 in france and italy. of course there's a reason why it's not so widely used, that being a customer department way lower than skoda (it also applies to lancia, for the moment, there aren't 20/25 lancia rally2 in the same entry -let's see if they will develop a big department with time- and also toyota doesn't reach the same numbers, even if it has a very good structure, of course).