It will always be available.
The question is will it be available at the price you are willing to pay?
Eventually the days of really "cheap" surplus 7.62 X 54R ammo will go away as those surplus stocks are depleted.
Remember, the reason it is "cheap" now is that some country, somewhere, decided that the ammo in question was no longer "needed" due to temperature exposure, age, reduction in military capabilities, obsolescence, etc. There maybe millions of these rounds produced and in storage somewhere, but until they are "surplused" by the country in question, they are off the market.
I remember when you could get surplus Portugese .308 ammo for less than $.20 a round and I thought it was steep then.
When the surplus stock is depleted, you will start paying market rate for the materials and labor used to produce the rounds. (Like we do for XM193 & XM855, XM80)
Then the price will be pretty much identical to .308/.30-06 but without the economies of scale of those rounds as produced in the US.
That is assuming there isn't an Executive Order/BATF interpretation that changes import status before then.
That is one of the things to keep in mind about offshore ammo supplies.
Will it happen? Who knows.
Can it happen? It has already with some types of ammo.
If you have a gun and you like the ammo, "buy it cheap and stack it deep", as the saying goes. But don't buy a gun of a specific caliber due to current lower than market prices on the ammo, expecting it to last forever.
It won't.