Would it not be more to the point to ask whether I believe in an "ideal" computer
No! It isn't more to the point. You may believe that all physical things are subject to entropy, and that therefore no physical computer could last forever, but you should still be able to talk about whether or not some program *would halt* if it were allowed to run forever. Look at the following program:
1: X = 1 2: X++ 3: if X < 1 then halt 4: goto 2
This program would clearly never halt if it were run on an "ideal computer" - and we can recognize that fact even while believing in entropy and the physical impossibility of running a program forever, etc. So the question is, for every possible finite program, do you believe there's a fact of the matter about whether or not it would halt if we *were* able to run it forever?