The main purpose of Modern Format was to give Splinterlands an affordable entry price to new players

I don't consider that to be the main purpose of Modern format. Pretty much every trading card game I am aware of does something similar, because over time as more and more cards are released it just becomes too overwhelming to be able to choose from that many cards, and it's extremely difficult to maintain a fun and balanced game with that many cards as well. With Splinterlands it's even more pronounced because you have a limited amount of time to choose your cards.

Also, how does it makes sense to sell more cards to mainly cover the cost of the creation of the new cards.

That's not the case at all. Rebellion brought in over $3M in total, half for the company and half for the DAO. It cost only a fraction of that to actually build the set. Card sales are what pay for everything and allow this game and economy to exist and continue to operate.

That being said - I'm interested to hear how you would suggest doing things differently.