I feel like there's two sides to this. On one hand, I agree with Zulip: don't make promises you're not sure can keep. On the other, you also shouldn't believe promises if the person making them is obviously not in a position to guarantee.
Example: suppose I offer you free email hosting "forever". Should you believe me? What if I go out of business? Well, in that case, I can at least ensure that your email address gets transferred to a different provider, so I'll still have kept my promise. So maybe this particular promise is believable.
But that only works because email is more or less a standard so there are many providers. Suppose I offer something that no one else does. Can you trust my promise that it will be "free forever"? Clearly, if I go out of business, then no.
Can I at least make a conditional promise that it will be free "as long as I'm in business"? But suppose I'm a month from bankruptcy, and my accountant tells me that getting rid of my free tier would save me. Surely, it's better for my free users to lose service rather than ALL my users losing service. So, unless you're sure I'd never be in that situation, you shouldn't believe me when I say "as long as I'm business this will be free".
Okay, how about a vague promise like "this service will keep its free tier around unless the business is in a desperate situation of some kind"? That's a promise that I could indeed keep, if I decide it's important enough... unless... how sure are you that I'll remain in full control of what is currently "my" business? What if I take my company public? I might then be kicked out by the board. (It happened to Steve Jobs, right?) So either my "free forever" promise means I'm not allowed to go public, or at least I need to do some very careful legal acrobatics to ensure that the board can't go back on my promise, even if they kick me out.
Still, if you find yourself needing to break a promise you made about "forever" a mere month after you made it, you should probably at least apologize instead of just hoping that no one remembers that you ever made such a promise. Chances are, they will indeed remember. If "it's the right thing to do" is not enough motivation, then do it for the brownie points, you'll get more of them this way. ("Who cares about brownie points?" you ask? Well, clearly, if they weren't worried about brownie points then they wouldn't be playing this weird game of "let's pretend we never said the forever".)