Increasing the team number doesn't affect the quality of the tournament in any means, only the tournament schedule gets extended. Among the entire sports league played around IPL is among the top 10. This has come as a result of perfect organising. In between some issues arise and two teams weren't allowed to play the IPL. If it is a drama this could've never been this successful.
But, I feel the more a tournament gets extended its importance decreases. Now after these two teams the tournament would bypass 90 days schedule. The only benefit would be that we will be able to see new talents that were left out by the previous franchises. The longer IPL runs the more BCCI will earn that is the motto. It will also affect international schedules as there would be less space to accumulate international matches.
We also know that IPL has a really big effect on ICC itself. And BCCI gets really huge amount of money from IPL. But India does utilize the IPL really well to find some young talents and later make them world-class players. A lot of cricketers in the Indian national team have come from IPL. But with 10 teams, it is going to take really long for IPL to end this time. The international cricket schedule will also be affected by this as you said.