I think it might happen in a span of 10 years. The way IPL is growing right now I do not see this as an impossible task. .
For the relegation system or the IPL window to expand IPL needs to grow drastically in terms of teams, hypothetically let's say 10 more teams in the next decade.
But If we analyze the IPL expansion so far, then there is virtually no chance. I've mentioned earlier that the next 4 year deal is locked in based on 10 teams so there would be no expansion. There might be some chance of 12 teams (2 additional teams) IPL after the 2027 cycle.
I personally think it can be done even with 15 teams. I will also say that India as a country has enough players to actually have 15 total teams in the IPL. Money is also not a problem so they can bring in foreign players as well. But sometimes we see the big investors tend to invest in the teams that are well established. So that can be a problem while trying to increase the number of teams in the IPL. But I believe if there is a relegation system the IPL is going to be more interesting.
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I think it might happen in a span of 10 years. The way IPL is growing right now I do not see this as an impossible task. The only problem is that I do not trust cricket to actually stay alive after 20 years. Unless ICC changes some rules. Cricket is not going to survive only throughout the Indian and Pakistani players, if you know what I mean.
Cricket will be well and alive as long as people are interested in them and the reason IPL and India is able to generate huge revenue than any other country is because of the fan base and the ability to fill any stadium whenever a match is played and the TRP rating they are receiving. Australia, New Zealand and England along with the other Asian countries are the major teams and it will be like that for the decades to come and the present fan base wont be diminishing.
It is true that the IPL is self-sufficient. And as long as the Indian people are interested in the IPL they are going to continue to improve and flourish. Even if cricket is somehow dead in the West I believe it is going to be alive in the subcontinent and the IPL will also do well. But we all know that things can change very quickly. And I also said that-
"cricket is not going to survive throughout only Indian and Pakistani players"
By that I meant cricket is not going to survive if the associate nations keep making national teams with Indian and Pakistani players. They will have to find players of their own.