"I'm also a realist that people are turning up and watching T20 not just at games but also on TV - society's changing, isn't it? People don't have four or five days to commit to test cricket. They might watch the first session, and the last session on day five if it's tight, but they're not going to then you strip it back a level as well and you think domestically, how can teams around the world afford to even exist?"
McCullum predicts a huge shift in the game where franchises will have the say over whether players will be allowed to play internationals.
"Long long-term, I see a T20 franchise as owning players, and I don't see them releasing those players to play for their nation in a test match," McCullum said.
"I don't buy that you need residual [test match] skills to be able to then transfer into T20. To me, they're played with the same instruments but they're completely separate games. I think once we do separate it even more, then the skill level of T20 cricket will go to a whole new level altogether."
McCullum's Royal Challengers currently sit in seventh place on the IPL standings with four wins from 11 games.