A couple of really good suggestions that would certainly add something to the game. Making the playoff cut off date occur sooner or placing a minimum game requirement for playoff qualification. I can see the debates on this firing up!
I've been burnt and seen it happen to plenty of others as well (Beechy comes to mind instantly). Managers are able to buy in huge salary teams, within such a short time before playoffs, that they can easily save enough money to get them through finals without going bankrupt. This has been especially so of late because those high salary players can be bought at such ridiculously cheap prices. Most teams can't afford the weekly wage of a $150k player in Div 2, so more people compete for players between $50k-$120k and pay more trying to purchase those players in bid wars, in comparison to those purchasing the higher tier players due to less trade market competition. There is some risk involved but more often than not, these teams can bank on achieving their goa byl using this tactic.
If they had to keep these high salary players for a longer time, it would make them have to plan ahead a bit better. They would still be able to save up, it would just take longer to accumulate enough cash to make it work. Quite possibly this may deter at least some managers from taking this easy option to a title.
Part of the problem is also that in order to promote and stay up, you need a team worthy of the division before you get there. So it really is a matter of timing and having all the dominoes in place. It's a tough one really and unfortunately I don't see any changes coming in this regard any time soon.