FB11,
A good enough bring back to fight a decent war and a good few campaigns, and deliver an effective service life. No useful bring back at any stage of its career? Sorry, can't let that one ride.
F-35B bring back margin is squeezed because the requirements are tough. Doing a VL with full internal weapons plus a lot more fuel than the Harrier ever had, at high temperatures, with an intentionally degraded engine (allows for in service thrust loss), plus another weight margin, plus a maximum weight engine (GFE so LM have to be given a weight the US government can live with until out of service) - well, you get the picture. If you want some fun, go and look at the bring back margins for some well known CTOL aircraft - you might be surprised.
I'm not trying to sugar coat this. Powered lift is always going to be tight, unless the users want to throw away a lot of capability. But the F-35 team have not gone into this blind. The engine will get more powerful, (F136 would be ven better than F135) but I doubt that airframe weight will go down.
Hope this helps
Best Regards as ever
Engines