MiG21s came in 71 and have retired just now. After >50 years.
SU30 is likely to be in service for another 25-30 years. MRFA program is still on. Whatever comes in, would fill the gap to an extent.
AMCA is the first iteration of 5th Gen. It is not the end of that program but a start. It is likely that the next version will start taking shape as AMCA is getting inducted or maybe before that.
Another aspect is requirement to have only stealth aircraft as the mainstay. Is it really required? Even US is indenturing F-15s now, which would last them for many decades.
AMCA, LCA MK II and later versions of these are likely to do service for India in the coming decades.
All these is appears to be based on assessing the current government and their inclination.