You don't want to put all your eggs in one basket. We face a formidable IADS on our Eastern border and the IAF has been practising rapid dispersal tactics like operating from highways. By that same logic, smaller, cheaper aircraft provide redundancy and risk reduction.
Even the mighty USAF dropped the much more ambitious E-10 airborne battle management plattform (combined air+ ground surveillance) and settled for Wedgetail.
USAF’s premier airborne battle management systems often leave enemy forces nowhere to hide.
www.airandspaceforces.com
Granted, they are investing in space-based ISR but so can we and at much lower cost, at that. The Anvesha series of sats is just one example.