1- People buy Bitcoin because it makes them profit from it
2- Bitcoin makes profit because it makes people buy it
It's a circular argument, and creates a perfect closed system, no other "product" can make it, in the center there is greed moving this perfect circle
Bitcoin is not a closed system when it comes to economics because there are lots of factor affecting it. The increase in price of Bitcoin is not as simple as people buying because of profit and price increase because people are buying it. Remember in this process people also need to sell in order to bag in profit.
Outside from selling, there is also emotional and logical factor included. There is also the need for adoption, marketing and hype which is outside factor that affect Bitcoin performance in the market. There is also this manipulation that greatly affect the fluctuation of the Bitcoin economy.
And why it's impossible to copy bitcoin's proof of work, meaning that only bitcoin's proof of work is valid:
If many people wanted to buy another PoW coin instead of Bitcoin, the price of Bitcoin would rise, meaning it would generate more profit, which would make people buy only Bitcoin. Therefore, it is not possible for people to buy another
proof of work coin instead of Bitcoin because this would make Bitcoin's price increase generating more profit, which would prevent competition.
Each cryptocurrency have their own community and market performance, emerging cryptocurrency can copy the template of Bitcoin but it cannot duplicate exactly the effect of Bitcoin because of the said different market audience.
People are free to invest their money to any POW that emerge in the market since it is a free market and no one is controlling people on where to invest their money. Whether we acknowledge it or not, there is always a competition between cryptocurrency since this competition is created the moment a certain new cryptocurrency is created, it is that these later cryptocurrency is unable to best Bitcoin.
a- If people starts to buy other Bitcoin competitors, it would make the Bitcoin network less secure
It is not the buy and sell of Bitcoin that made it secure but the hashes produced by mining Bitcoin. So it would make sense if you stated that if the mining farm that were mining Bitcoin shifted their hashes to other cryptocurrency, it will make the Bitcoin network less secure.
c- Then if the Bitcoin network becomes less secure, then all parties, buyers and sellers, are forced to accept a higher Bitcoin price to make the network more secure, and this price increase discourages/block people from investing in competitors
Again it is not the bitcoin market of selling and buying that make the Bitcoin network secure, Bitcoin can be secured even it price collapse as long as the miners remains, after all it is the number of hashes mining to solve the block that has the direct affect tot he security of Bitcoin network.