That's reasonable, but I like it this way for two reasons: better heroes appear over time, so this way makes for an interesting decision, where you must decide you want to pass on later heroes to get and develop the early ones, OR pass on some early ones, making your game slightly harder but leaving slots for the good ones later. If you could always get offers then there would be no real downside to always accepting every hero, which is less interesting - I like that there might be some late-game hero that I've never even seen! Also, once I have four heroes that I am happy with, it's nice that the game stops bothering me about it.