In answer to people saying "modern cars use glued panels" - yes, they do, but they are designed from the very beginning to do so. A lot of modern cars are made from aluminium, but you wouldn't get a sheet from hobbycraft, mastik it in and call that a suitable replacement for steel on a 20yr old 306 either. Morgans come from the factory with an ash frame, would you break out the woodworking skills to patch up your hatchback?
The structural rigidity of a monocoque is vital to it's crash protection, load transfer and torsional rigidity and therefore safe handling. Yes, the boot floor is part of that structure.
They are designed very thoroughly over a number of years, with millions of pounds of development & testing. If you think you can replicate this with a grinder, a scrap yard cut-out and some tiger seal, I suggest you contact PSA, VAG, Ford etc and earn your millions overnight.
Messing around with this because another car manufacturer uses glue/aluminium/wood/carbon fibre could compromise the car's integrity. This is the reason why MOT testers look for rust, check repairs etc: cars can easily become
unsafe.
As for the whole "nanny state", "retro rides is full of negative people" argument; if you ask a question on whether something is suitable and experienced, knowledgeable people say it is unsafe and not roadworthy - why is that negative? That's called
constructive feedback, there is a massive difference.
Ask the same question on any online forum and you will get the same answer, because it's based on law, fact, experience and knowledge.