You are right that some old coins still exist but the cash flow such as the market trend is not as strong as it used to be the truth is that as long as they still exist and people are still using them for transactions, it is still worth it and it would still be in existence as long as it is reliable and has proven records in the past.
I am not sure if people used some old coins for transactions. Most of people only hold the old coins for investment. Sure, as long as there are people who hold and trade the old coins, they will still have values. However, the old coins must have innovation, or the developers need to develop the projects and make the coins to have vital function. If they don't do this, the old coins will lose their values gradually. So kindly avoid to buy old coins which have no progress or no development.
I appreciate what you said about old coins that are not progressing and cannot develop, so they are not worth holding because they will be detrimental later.
But there are still many old coins that are very productive and can contribute to their holders because they are always used as trade and can generate income.
Actually, old coins that still have potential are certainly very good for investment and trading because they are usually able to develop themselves so they can contribute.
Don't always trust new coins that arise from unclear projects and are very detrimental later.