Let's break it down -
If you delay promotions, you get 4 $300+K seasons out a player. Add 1 arb season, That makes 5 seasons per player before you have to trade them for whatever you can get or you release them.
25 players on an ML roster. If you nail this system, you'll have to come up with 5 ML rookies every season.
If you want to have a good team, lets' say 4 of those 5 have to be above average.
Where do you get these 4 above average rookies every season?
Promote 1 above average IFA every season. (Which you've already seen can't be guaranteed.)
If your team is always good, you're never drafting low. So you'll have to budget and know how to draft and get a bit lucky to have your #1 draft pick be above average. Let's say that happens every season (even though it won't). That's 2 players.
If you are in a world with its share of less-that-expert HBD GMs, some seasons you'll pull an OK ML player with your #2 pick. Some seasons you won't. And some years you'll pull on OK player, but they play positions that don't match one of your 5 openings. Hard to have a good team with crap D. And you need the right balance of pitching. 7 short relievers won't work. So let's say every other season your 2nd round pick is that 5th player you need ever season. Not average, but OK. Roll / bench player.
That leaves you needing 2 or 3 players every season, 2 being above average.
You'll have to sign 4 of those 5 2nd season arb guys to trade them. I'm assuming the 5th player you're letting go is the below average one who won't have any trade value with an Arb 2 salary, so you released him.
Looks like you'll need to find 2 GMs in your world, every single season, willing to trade you 1 above average prospect for 2 players who are also above average, but about to get expensive.
How hard can that be?
And don't forget if you delay promotions to get that 5th cheap season, you'll play 24 games every season with 5 $327K players on your ML team that nobody else wanted. Best of your MinL non-prospects. Rule 5 players. Dregs of FAs. That's going to put a dent in your playoff chances and cost you and extra $1.5M / season in salary.
Please report back in 5 seasons when you've got this up and running.