In terms of financial stability, it's important to note that the MLS is 10 years older than the A-League, and that extra time has allowed the MLS to grow to a place where the A-League should be aiming for. The A-League and it's clubs are still very young, but attendances and financial stability is growing across the league. Just like in the US, the Australian sporting market is extremely saturated, and the A-League is still working on expanding the number of people following it and attending games, which the MLS has a 10 year headstart on.
I agree wholeheartedly that the MLS is at a higher overall quality than the A-League for these reasons, but I'm not so sure about your comment that it is NASL level. I've never watched a NASL game so you are obviously in a better place to comment on a comparison between the two, but I don't think the difference in quality between the MLS and the A-League is as great as you think. Having watched nearly all of the Timber's MLS games last season, I think that they would have found it quite hard to beat some of the best teams in the A-League at the same point of time. Brisbane Roar at their peak or the current Mariners squad would give last years Timbers side a run for their money. I've never watched a TFC game, but from what I hear they were even worse than the Timbers.
In summary, I think this is a fantastic loan for Brockie, as he will be playing against higher quality opposition, but the quality won't be so far above him that he will feel out of his league.