in fact Bitcoin is speculative because it's decentralized - if it was something like a stablecoin managed by a company, it would be very centralized and tightly regulated, so there won't be any free market.
All cryptocurrencies/tokens are speculative right now because they're not being used (comparatively to other solutions that aim to solve the same problem, albeit not in a decentralized way). I don't think decentralization inherently has anything to do with how the entire market is speculative. Investors/traders are speculating that Bitcoin and other DLTs will have a real-world use case that will drive their value in the future.
there is a big difference between calling bitcoin speculative and altcoin/tokens speculative.
when you look at bitcoin you can see that it has been used as a currency for a very long time and in thousands of different places both online and offline. the payment processors are processing billions of dollars worth of bitcoin transactions, and that's just processors we have no data from those merchants that accept bitcoin directly!
so when you say bitcoin is speculative, that speculation is about future
increasing success and adoption of bitcoin.
but when it comes to altcoins and tokens that have no usages and have never been used for anything anywhere other than exchanges, being speculative in that context means being something people
gamble on.