I didn't think the op was unreasonable to want to leave an hour later in the circumstances, but I was basing that on my own experiences of renting cottages/villas.
We're a large family so we haven't always made it out the door before the stroke of 10. Because it's not, as some people suggest, a simple matter of doing it all the night before and then throwing a few things in a bag the next morning. There's the tidying (replacing all those unnecessary cushions and ornaments which you've popped in a cupboard out of the way) cleaning, emptying bins, fridge etc most of which if you did the night before you'd have to do again the next morning when you have kids and dogs. We have however always brought everything down from the bedrooms so that the 1st floor is completely clear and ready for the cleaners to start on on time. I realise some of you think this is totally unreasonable, if not illegal, but we've never had anything other than smiles and pleasant chat from the cleaners.
Last year in Dorset we were packed up on time, cleaner arrived and then sat in the garden with us for about half an hour chatting 🤷♀️.
In Greece we've had the cleaner turn up early (horror!) and then following me round almost slapping my hands to stop me doing last minute cleaning because 'you're on holiday!).
So my own experiences are that the 10am deadline is a bit flexible (though as I say, we try our hardest to stick to it). We've also regularly turned up early at holiday cottages on the off chance that we can get in early, which we invariably can. So I've always had the impression that 10am to 4pm was more than enough time for turnaround and that the owners just erred on the safe side by allowing a bit extra.
But I am absolutely horrified by some of the accounts on here of the state people leave properties in. I had no idea so many guests could be so disgusting and thoughtless. Sounds truly grim. I'm horrified.
Also interested in the comment from HolidayLetter that it's annoying when people strip the beds and remove the mattress/pillow protectors. I always do that if asked to strip the beds before departure. Never occurred to me not to remove the protectors and that there wouldn't be clean ones put on for each new guest! They're only covered by thin linen which won't stop sweat and other bodily fluids soaking through ....