I partially agree with you. The main purpose of IPL is to provide funds for BCCI. And at the end of every ipl we see BCCI got an huge amount of revenue from ipl in an era. You said that new players are being sought in the Ranji Trophy. You are right.But whether those players are suitable for the national team is tested in IPL. Because all the best players in the world play in IPL.
IPL is just like a mini-T20 World Cup, because you may find the best players from all over the world. And that's the reason why it may be logical to increase the overseas player limit to 5. It will remove the useless players such as Kedar Jadhav, and replace them with quality international players.
During IPL 2020, I noticed a large number of quality international players, who didn't got enough opportunities due to the 4-player limit. Some of them are Mitchell Santner, Imran Tahir, Sandeep Lamichhane, Mujeeb Ur Rahman, Hardus Viljoen, Mitchell McClenaghan, Adam Zampa, Mohammad Nabi and Fabian Allen.