Yes, they are perfectly valid.

Paper tickets will become useless in 2021.

November 2019 is before 2021, or am I going senile?

No, ever since magnetic strip tickets were introduced in the last century, existing tickets have always remained valid even when there is a fare increase.

I hope they merge the functions of the Easy card and the Navigo card, Also, I dislike the effect of requiring a couple to buy two Easy cards and separate carnets, while paper tickets allow sharing a carnet asnd no extra cost for cards.

People will always need individual transport passes. Can you imagine the effect if a ticket checker is told, after an argument for example, "no, I don't know that person"?

In New York City, multiple fares stored on a metrocard can be used for up to four consecutive riders at a turnstile, allowing discounted rides to be spread over a whole family. The paper carnet tickets can similarly be distributed. A card that is personal cannot allow this. Charging a fee to have the card is just another tourist tax.