So, I'll ask you once again: where do you see value in the bitcoin system if not a single bitcoin holder can return even a dime worth of investment without new investors?

I still benefit from censorship resistant transactions if no new participants join.
I still benefit from psudonymous levels of privacy if no new participants join.
I still benefit from the strength and security of the network if no new participants join.

Because the network itself is a tangible resource.  Real people running software on real devices.  Ensuring that every single transaction in every last block conforms to the rules of the protocol.  All of that infrastructure exists.  And you need bitcoin (the currency/asset) to benefit from Bitcoin (the network).  The value is real and it's only the willfully ignorant who are unable to see it.

You can benefit from the security and the anonymity of the system. One crypto owner can buy a good from another crypto owner and therefor both benefit from the bitcoin system. There is no need for a new buyer to enter the bitcoin ecosystem.

Back to the rest of your question.  I can send you $100,000 worth of Bitcoin without a third party interfering with our transaction and requesting documents for identification and proof for source of funds.

I can move $1,000,000,000 abroad without making anyone notice that I did.  It is a transaction stored in a public and transparent legder, but pseudonymity makes it so that there is no clear evidence of who I am and with enough privacy and care, I can make it close to impossible with the current knowledge, information and technology to link the transaction to my identity.

I can purchase a ton of goods using Bitcoin directly, at my own will, even by physically shopping and checking out in a similar way to credit card payments.  Only without banks.

I can store information directly on the blockchain.

I can use my own resources and energy to 'generate' or, as we say, mine Bitcoin at my own will.

These are just a few of the things I can do with Bitcoin.  This is how I benefit from it.  Tell me where exactly is the 'new investors' part in any of the five scenarios I have mentioned above.  Exactly, there is not.

Then I think value is actually subjective.  If all of a sudden nobody thought USD had value, it will not have value any more.  If all of a sudden everyone thought Bitcoin was worthless, it becomes worthless.  We as a society decide what is valuable and what is not, otherwise precious metals would not become worth less during peaceful times and worth more during panic.  Its value changes depending on a number of conditions.

Bitcoin is just another manifestation of the same debt.
In the past people have been using shells, or silver, or gold for the same job. Why can't we use Bitcoin? Because it's... digital? Or maybe because the governments cannot alter the value at will with inflation?

Debt in crypto is the appropriate amount of US Dollars (or whatever other fiat currency) that was used to buy the bitcoins in the first place. This is what backs the quantity of BTC.

Besides, transaction establishment existed long before debt became a thing.