I think many individuals in this area have felt some form of what you're saying. Altcoins obviously come with substantial risks and a lot of projects have failed due to centralization, poor management or blatant scams, The Terra Luna collapse was a tremendous wake up call. The fact that many of these coins are run like startups rather than decentralized networks makes them open to a lot of manipulation.
It is not many, it is basically all of the top coins excluding Litecoin and XMR. They are all run like companies. Some try to fake decentralization and create a foundation to run it, but it is the same thing. As with most of their features, even their organization structure is about faking things.
i am surprised with the topic and more informed by your post.
but does it mean that BTC is the only reliable crypto due to its decentralised system/setting?
This is disturbing for me because i invested in multiple crypto and looking forward to the bull season.
Perhaps i might need to sell off my altcoin and buy just BTC? or what do you think?
When you are investing in most altcoins you are buying centralized coins that pretend to be like Bitcoin. That does not mean that you won't make money there. You can, but at the same time you are supporting fraud and scams. Then again, most people who are invested in altcoins are like that so you can do whatever you want.