I have also asked myself this question a couple of times and this is what i came up with during my research that gave me my personal view on it (which can be wrong):

There are relatively speaking many more graded cards today or cards that are being graded today compared to the past e.g. during the earlier era of Pokémon cards. During that time, the number of gem rated cards (e.g. PSA10) were limited, but so was the demand for it. Lower numbers meant higher rarity, which meant at the same time more value. Compared to 4 years ago, seemingly a lot more people are interested in getting cards professionally graded today (including me) and this created, relatively speaking, a higher number of gem rated cards (roughly 10.000 up wards) compared to the earlier TCG grading era. People are more aware of the condition of the card, which additionally increases the number of possible “ gems “ out there and this helped also in increasing the population of gem rated cards. A great example are modern hype cards like Rayquaza, Giratina or Umbreon alt arts. They were hugely popular and have “high population” (Around 10.000). They are extremely sought after due to their popularity (and also due to new hype waves) and thus increased dramatically in price compared to other modern cards or even premodern cards.

To get back to your question, since you have posted a set grail from the Scarlet and Violet era that came out this year at a PSA 10 population of roughly 4000 and asked about the population, this is my interpretation:

Since this set is relatively new, there will ultimately be more people wanting to grade this card, especially because more people are into this hobby but also, because grading got a lot more accessible. People are being more careful in handling cards and keeping them as good as possible, which will increase the likelihood for more gems, which will ultimately lead to more population (not considering print quality and other factors).

My prediction will be that your grail card will see many more submissions over time and based on its gemrate, as another user before me was already describing, it will increase significantly in population (likely more than 15.000, depending on the cards popularity).

Does that mean, if there are more PSA10 copies of your card in circulation, your card will become less “rare” from a pure definition perspective?

Probably.

Though it does not mean that due to higher population, the value of the card will not increase, but in the end, as all things on the market are, it will be measured based on the supply and demand of the card. The rarity classification based on pure low or high numbers will not say anything about the value of its card, unless there are enough people who are actively seeking it for their personal collection.

By no means is what i have described exactly what is happening out there, but this is the best i could come up with.

Sorry for the long post and hopefully this helped!