Your line of thinking is what companies love, it is what allowed these companies to generate increasing profits every year. Are you actually advocating that businesses are allowed to raise prices all the time regardless of reasons? That's how health care insurance got to where it is now with hospitals charging 10K for a simple IV injection where they actually cost less than a dollar. I suggest you check their sheets, if the inflation was a problem, their expenses would've jump higher to match, they don't and sometime, they're lower. Also, not all expenses increase in cost ever year; such as bandwidth costs (wired transfer between each endpoint) is constantly dropping for the past few decades. Radiowave spectrum is not unlimited, so the expense is in adding more towers but the long run is cheaper with more customers. So, it is not as simple as you make it sound and no, businesses do not increase prices all the time for inflation, they often do it for different reasons, especially if they're for-profit with shareholders.
Inflation has zero to do with this, they're not losing money, they have a different unlimited plan that has the same "unlimited" perk with slightly more limits (with more hidden fees) that the grandfathered plans don't have. If ATT can't afford the "inflation" and the expenses, they should not have an unlimited plan in the first place and they shouldn't have a cheaper one that is more customer-hostile than the said grandfathered plans.
Oh by the way, ATT is already been fined by the government about different actions they took against the grandfathered plans and FCC confirmed the methods they took before had nothing to do with network management but merely to enhance the revenue, just like what they're doing here. The only difference is, this is sadly legal.
They're not losing money on this, they're intentionally raising the price to get rid of the grandfathered members to switch to the other plans. That's just a business trick, it's a well known established process that works. Raise the price of a service plan to move everyone over to the cheaper but more limited plans. It has ZERO to do with inflation but a business decision made that's meant to generate more profits.