There are two problems with that:
First Japan has done a great job gearing up for 2019, but there’s a suspicion there’s not a lot underneath. Although rugby is Japan is very popular the standard below the international squad and arguably even below about the first 25 or so players starts to fall away a bit. The Sunwolves were admitted to SANZAAR’s Super Rugby tournament in 2016 but are about to withdraw for financial reasons.
I wouldn’t be surprised to see them go backwards after the Works Cup.
For international rugby the question would have then to be: would it bring Japanese rugby on? They did ok without it so far. Which brings one on to the other issue: world rugby is not run in the interests of developing nations and never has been. There was a plan drawn up by World Rugby which would have seen for example promotion and relegation from the Championship and Six Nations but it was voted down: admittedly there were various issues there, but it’s a bit of a disgrace that these competitions are run by themselves effectively as cosy cartels. Meanwhile the World Cup we all love but there are still too many mismatches: when Scotland and Japan looked like being cancelled there was a lot of dismay but less when New Zealand v Italy actually was called off despite the fact it meant Italy were thereby eliminated. But the sad fact is we all know Italy would have been thrashed in that game and that’s rather sad.
So in summary Japanese rugby desperately needs rather than “deserves” such a promotion. But sadly it seems unlikely.