Interesting Mr Doug.
IMHO, money assists with their performance, but not necessarily their development. The larger benefit would be funding for facilities and coaching - the latter has certainly helped Fiji.
Ultimately, those nations without enough internal momentum have to gain it through getting their
players professional through overseas contracts and then build their internal game based on the success those players can help deliver. This has assisted Fiji, Tonga, and Samoa, none of whom are big enough to develop players internally.
Looking at Uruguay as an example, they have parameters that simply adding foreign players won't overcome for proper long-term growth: the population is just over 3 million, of which nearly half live in Montevideo, and while it is considered an honourable game there, it is third behind soccer and basketball.
Here is the wiki entry:
https://en.wikipedia.org/wiki/Rugby_union_in_Uruguay
The rugby competition (centered around the city) therefore needs to be able to produce good players on a regular basis, and support them at least on a semi-professional level so that the depth can be built, and competitions can attract sponsors and start funding themselves.
That money and experience comes with success for the national team (much like here in Australia), and that means having more than 4 professionals available to them for big shows like RWC.
Still, countries like Ireland have about 4.6 million, and have soccer and hurling and gaelic football as competition.
BUT Ireland have over 150,000 registered players, and Uruguay have not quite 6,000. Throw in the fact that guys like POC, Sexton et al can earn a very healthy living playing in Europe, with their club and international competitions on the doorstep. Meanwhile Uruguay have to rely on a very weak South America for rugby, which is limited to their home countries.
The balance therefore has to be struck between getting good players at the top level to give kids aspiration, while funding the grassroots to make sure they can stay in the game.
I think this RWC so far has shown that the amateurs are accelerating faster than ever before, while the Tier 2 nations are also improving, if at a slightly slower rate.