...Before reading this thread, if I were seated and someone came along with a ticket and seat reservation then I would get up and give them their seat, I would also expect the same action from someone occupying my seat if I had the ticket with the seat number reserved printed on it....
It falls down when you consider that the vast majority of passengers use non-Advance tickets and use the railways as a convenient mode of on-demand travel and not as an airline.
I'd not expect a parking space to be reserved if it didn't have a notice on it to that effect, and many other similar examples could be given. Anything that is available 'on demand' and does not require pre-booking should not force someone to have to move seat with no warning potentially every 5 minutes!
Surely the problem here is that if there are no announcements made....
That is the problem. Guards should announce this.
Hmm ... on a number of occasions on DB I have taken a meal in the restaurant car before going to my reserved seat. I have never had any problem claiming the reserved seat ... maybe that is just down to the natural politeness of German rail travellers.
Or you being polite when asking

If someone who has a need for a seat asked me nicely to give up their seat as a favour because they needed it, then I am far more likely to give up a seat than someone who rudely demands it.
I would expect that. If I have a little card in my hand that says I have seat C18 booked on the 15:26 between Leamington and Manchester, then if I get on that train I'd expect to be able to sit there. If the TOC hasn't labelled the seat, then it's unfortunate for the person sat there, but surely my holding the little card should be the decider?
Say you live in Penzance, you have to urgently get to London to see an ill relative. You buy a ticket to London for immediate travel. There are no reservations on the train. But just before departure someone turfs you out, you'd move? And at St Erth, someone again tries to turf you out, you'd move again? And at Cambourne, someone else wants your seat, so you'd move a third time? The train then gets to Redruth, the same happens, you'd move again? Then you get to Truro... I won't list the other stops, but, you're telling me at every station you'd be prepared move from one apparently unreserved seat to another unreserved seat? Seriously?
... i got on the train and went to my seat and a businessman was sitting there.
so i asked him to move ...
Were reservations provided on the train and your seat had your reservation details on it and you held the coupon for it? If so, clearly you were in the right. However if an equivalent alternative is available it's best for the Train Manager to use their judgement to avoid confrontation. If a suitable seat was not available then the Train Manager would need to either ensure one was provided or else you would have a valid complaint in which case I'd then contact Customer Services... but only after the TM was given every opportunity to provide a suitable alternative.
And, of the members of the public, which should be inconvenienced for this - the passenger who has taken all steps available to him to ensure that he has a seat for his journey, or the passenger who has not but has bagged the seat anyway?
And, of the members of the public, which should be inconvenienced for this - the passenger (A) who knew in advance they could only take that train and got a reservation in the anticipation it would be honoured but on the understanding it could
not be guaranteed (as per NRCoC), or the passenger (B) who bought an appropriate ticket to allow flexibility of train with the hope that they could get a seat but an equal understanding that it could not be guaranteed, who on boarding discovered that no reservations were in force on the train and therefore had no way of knowing which seats were originally reserved and which were not and therefore sat in an appropriate seat on the understanding that this was effectively an unreserved train (which most trains are!) and therefore the reasonable expectation that someone would not randomly ask them to vacate their seat at any station en route?
Not quite the same question when you consider it like that, is it?
Remember there are statistically far more people are in the position of passenger B than A.