Stealing is illegal always , but in your case and question its hard to locate who's the real owner of that btc , so it's not your fault .

I won’t give lecture on morality because we all know that it’s wrong but let’s discuss it as a case. I think, yes it’s possible but it’s not going to happen in next 1,000 years for sure and till then crypto currency might have some totally different technology that would make security at its peak level. There is near to -1,000% possibility of the probability of having bitcoins in newly created address.

If that's gonna happen I thinks its pretty sure that it is illegal. It is a kind of stealing if you will use that bitcoin. It's someones earnings and not yours. Maybe you can search for the real owner of that address and tell him/her that you shared the same address. Maybe he will give you a bounty for telling the truth. And also you will have a +credit to your good deeds.

Indirectly, you are supporting and suggesting how not to be caught. It is not fair. We need to feel our responsibility to return any accidentally received bitcoins. It must be one of basic human behavior, we need to teach that to our next generations so that they will have a clear crime free future for better peace of mind.

Yeah its just a system fault and we can not consider it as stealing.. you are lucky if you had them.. but i think its impossible to happen i never heard it here before maybe they are receive some bitcoins from  unknown someone.

No, it is still stealing. The question is not who is the owner of the address, but who is the owner of the BTC. As many, many people have already said, the BTC was put into that address  by someone and that person is the owner until they decide to transfer the funds to someone else either by sending them on or transferring ownership of the address containing them.

All of you crooks who think otherwise, the same thing could be said of a house, car etc. If I go to the locksmith and have him make some random keys, and I try them on some random cars and houses and one of them matches (which has a lot better odds than the bitcoin example btw), does that make me the owner of said car/house? No, of course not. You may own the key but not to the contents or object it unlocks.

Yes i can never be a good thing if you are taking some thing which does not belong to you.Yes it is stealing and stealing can never be a legal activity.
And with bitcoin having really a good value in the market,then stealing bitcoins is 100% an illegal activity,no matter how you doing it.

just left the bitcoin there, create another key and that's it, you don't need to find the owner to say anything... that's it.
or well, YOLO, spend all and gg.

The only thing I can say about your question op is that stealing bitcoin is always illegal and never will be legal but if by some miracle you happen to generate a new address that belongs to somebody else and has a lot of bitcoin, it ain't your fault and it will be a legal reason. The one in fault if that happens is the system of bitcoin, thus will make it semi legal for you but if you happen to come across one, I only suggest that you leave it that way since maybe those bitcoins are hard earned ones.