The value of some coins does decrease every time and it is the same with bitcoin but bitcoin always manages to get out of the process of decline towards a new price improvement after the correction process takes place. That's why bitcoin is much better to invest in compared to other coins because even if a decline occurs, there will always be a process of improvement towards the next ATH.
To be honest, no matter how good Bitcoin and other cryptocurrencies are, I wouldn't consider them sources that can be used to earn passive income because a passive income should be stable and you should keep getting money as long as your source is live, but when it comes to Bitcoin and other cryptocurrency investments, you don't earn when the market is going in negative but you start losing what you have already earned.
For example, if you have $1,000 worth of Bitcoin when you are going to bed and Bitcoin is worth $30,000. When you wake up, the price of Bitcoin has reached $15,000 and you have $500 left, you have lost value when you were sleeping and a passive income doesn't work that way.
I agree that Bitcoin can be a great investment in the long run, but in the short run, you can't depend on it to earn you money constantly.