Miles & Smiles Upgrade Issue

I have been a Miles & Smiles member for about 10 years. For the last 6-7 years, I have held Elite Plus status. Due to this status, I am entitled to two free cabin upgrades per year. Additionally, I can also upgrade using the Bonus miles I have earned. However, I have noticed that recently the system only allows last-minute upgrades, meaning I can't upgrade until I am at the airport counter. I don't want to believe this is done to sell business class seats for cash until the last minute and only upgrading if there are empty seats left, as this would question the purpose of the Miles & Smiles loyalty program and the rights and miles earned by its members. I know dozens of people are unhappy with this situation but don't voice their complaints, thinking nothing will change. Please don't think this complaint is about a specific flight, because this situation applies to almost all flights. Why can't I upgrade a ticket I bought 3 or 6 months ago at the time of purchase? If there are available seats in business class, why does the system wait until the last minute? Why has this issue arisen in recent years when it wasn't a problem before? I would appreciate a detailed written explanation of these points.