I think you've got an misconception about monopoly and high popularity. Apart from Bitcoin, cryptomarket has a number of digital currencies, which is completely different from monopoly which refers to a sector or industry dominated by one corporation, firm or entity.
That Bitcoin dominates the coin market is obvious but it's still been riddled with problems. If in the future a new cryptocurrency is created, addressing Bitcoin current shortcomings, I think Bitcoin will have real competitor.
Yes, you are right. BTC is not doing monopoly business for sure but it is doing monopolistic competition. Monopolistic competition means there will be a little competition with open market and there will have lots of buyers and sellers. And BTC doing exactly like this. We have the chance to choose but we will go to BTC at the end.