From a mathematical perspective, of course its possible - the same way that its possible for a coin to flip "heads" 100 times in a row .. but I'm not going to sit around waiting for that chance, either.
In FM, factors such as declining motivation, fatigue, injury, and simple bad luck (red card, injury after 3rd sub, etc) would all tend to combine to bring winning streaks - and losing streaks luckily! - to an eventual end.
I haven't personally been in any danger

of completing an unbeaten season, but I remember a poster who was complaining about FM'06 or FM'07, whose basic point was "
Two seasons in a row I've clinched the title early, while unbeaten .. and my team have immediately lost the next league match. They should have the motivation to finish that rare unbeaten season, not lose motivation because they've won the title!"
So that's apparently another factor.
...
Note that the game is "easier" on a smaller database - if you run a Small DB with only England active, you can eventually reach a point where you have the 15 or 20 best players in the entire game on your team, and the whole thing becomes a bit too easy ..
If you play a Large database with England, Italy, Spain, and Germany active, the crop of "best" players will be too large for you to monopolize, so there will always be a couple of other good teams in your league, plus of course, in the Champions League.