If you get promoted, as you said, your current 4* players turn into 3* players and they get a year older. Thus they drop in market value and development rate. So for the same price you can get a one year younger player with same quality (quality remains, only the league star standard changes) if you wait until the start of the new season.
And with doubled money for tokens, you can for example sign now one 25 year old 4* with quality 24 or wait until the start and sign for the same money two 25 years old 3*, each with quality 24. The old season buy turns 26 and is rated in the higher league as 3* with quality 24. Conclusion: You used the token <-> cash function better with waiting a few days.
This example does not consider bidding wars at the start of the new season (which are to avoid), actual goals this season and I agree with you that if you find a player that is really for sale now (sale price far under market value) can worth an investment as well as quality (high growth) free transfers close to 4*.
But generally you get more quality for your tokens with waiting a few days.