This is certainly true. It seems to me the path to a truly competent sixth generation air dominance fighter would be a new fuselage for the F-35. The Navy did this with the FA-18 family and got the very competent E/F/G (Although they didn't cure the range issues the way they should have).I'm honestly not sure. The complexity of avionics and engines is very high (almost all of the 'moving parts'). The ability to engineer an airframe and new aerodynamics is getting easier over time.
If the airframe is holding back a willingness to update avionics, weapons, or engine then offering an improved airframe could be a relatively easy and necessary step for opening up the possibility of further investments in the program.
The question is the cost/capability calculation regarding avionics, weapons, engines... what to add, what level of capability, and what alternatives.
How difficult would it be to create the fifth generation fighter we should have had with the F-35 if it hadn't had to be spread across three different versions for three different services?
Having said that I suspect the F-35, exactly as it is, is the fifth generation fighter that India wishes they had right now. F-35 Pilots, almost all of them, insist it's an amazing aircraft.