They are not limiting the price but rather just speculating the price of bitcoin. A lot of people into bitcoin are here for the profit so if bitcoin reaches a million in value it will mean a lot for them in terms of profit.
That's what I was about to say, while we're also speculating its price based on a specific timeline; $150,000 till the end of the year etc. Although I understand the OP's perspective, it's also wise to set realistic expectations. Bitcoin's supply is limited indeed, however, there are over 14 million coins in circulation, taking into account those that have been lost forever; it's not exactly rare.
With that being said, speculating $1M in value is in a way, capping Bitcoin's capabilities, it's also a scenario we can't be certain it'll ever happen as well.
I think it will, though surely, it's all speculations and thoughts regarding of what - may - happen.