SE have never raised FFXI's subscription in the over 20 years that game has been running (even when it's sub numbers dropped enough that they were consistantly closing servers down), so they won't change that either with FFXIV (also, there is a whole group of players whose subscription model states "reduced subscription for life" - aka, Legacy Campaign Members - so it's simply not going to happen).

The cost to buy the game may rise, and new items and the like on the Online Store may cost more, but subscription will always stay the same (this is incidentally why also paying for subscription with a credit or debit card instead of using a prepaid time card costs less overall - the time card can and will rise in cost to buy but the subscription rate stays the same).