Now the question is, assuming everyone who ever bough bitcoin is holding their bitcoin in their private wallets, what will be the fate of bitcoin and the bitcoin network? Does holding bitcoin strengthen the bitcoin network? If no, are those who sell and more their bitcoin on-chain generally doing bitcoin bad?
Actually, the real reason why Satoshi Nakamoto invent bitcoin was for it to be used as a medium of payment alternative to the Fiat currency but over the years, investors found more value in it due to its appreciation in price which now made many people have the mindset of keeping it as a store of value with time.
To answer your question if holding bitcoin strengthen the bitcoin network, yes it does because since bitcoin hasn't gained mass adoption like the fiat currency made it become complex to be circulated from one person to the other because only few people own bitcoins. Most people that uses bitcoin as a means of payment still afterwards convert it to Fiat currency which is not healthy for the growth of the digital currency, rather than this practice of selling bitcoin, it would be better to leave them in your own custody while it appreciates and also give you the advantage of getting plenty of it in your portfolio if you are a DCAer. Storing your bitcoins will help you when enough bitcoins will be in circulation and everyone starts embracing it to make payments and other transactions, by that time, no one will be thinking of converting their bitcoins to fiat anymore. However, irrespective of the fact that we mostly preach about hodling bitcoin in the forum, it doesn't stop you from using it as a means of payment to another bitcoiner.
Remember, we can't hodl till eternity, therefore a time will come when bitcoin will start serving it original purpose of invention but for now, just HODL. However, the bitcoin network is already strong because each day transactions are recorded and miners are also busy in the blockchain so all these activities validates the bitcoin network.