the solution to the long delay between the end of the primaries and the conventions is to move the primaries later not make the conventions earlier
There's no easy way to force the states to move their primaries later.
obviously they have been trying to stop the frontloading of the primaries and how they have been creeping earlier and earlier, but this move of the convention seems like they are giving up on getting back to the way primaries used to be. In the 90s the NH primary was in late February. In 2012 it was Jan 8th.
I think there's a good chance that the new RNC penalties on states going early will be enough to push the starting date a bit later, perhaps with Iowa going in mid to late January. So a couple of weeks later than last time. But it's really hard to push it much later than that, to convince all those state legislatures to revise their election scheduling laws, and the national parties just don't care enough to force the issue.
But the timing of the conventions is really a separate issue. It was always the case until 2008 that the first of the two national conventions was held in July. That was the norm. It's 2008 and 2012 that are the odd cases, with both conventions being held so late. And as I said, that was done because of campaign finance law reasons that don't really exist anymore, since no one who wins the nomination of either major party takes public matching funds anymore.
