if you created your own altcoin.

you would then to make that altcoin work. need to mine it so that blocks are produced.
because you are just one person. with one cell phone. your altcoin wont make blocks for a long while. it would take months and years for all them missing blocks to trigger difficulty drops sufficiently enough for your cell phone to start mining successful and frequently mine a block so that you can make a payment to yourself or someone else.

by that point if you could get someone to be part of your network. they too could mine. and they would then value the altcoin based on how easy it is to acquire the altcoin by different means.

so if you were trying to offer your altcoin to them for $40k. they would decline because they are at a cellphone mining speed too, meaning 0.01c to mine fresh coin. so why would they want your $40k coin, when they can mine on your altcoin network for much less

..
now do you see why i mentioned the mining value stuff many posts ago

your altcoin might have the same features of protocol. but it lacks other features, like its acceptance by retailers

EG
they/you are not buying the serial number of a bank note.

if you had a bank note and decided to copy it(counterfeit) retailers wont accept it. trying to convince someone to accept it. they would have to evaluate the cost of your photocopy vs their own ability to photocopy. and their own risk of retail acceptance. and in the end they might offer you a penny for your counterfeit bank note.

or they would look at a genuine medium of exchange that has actual costs involved and ease of spending. and just use that one
even if the ease of use the genuine medium of exchange comes at a higher cost. because of its utility and features that make it better than the issues, headaches and risks of your silly altcoin/counterfeit bank note