Let's hope that the "modern rewrite" doesn't slow it down too much ;)
On topic - kudos for being free. There must be a threshold where a company that's "free forever" can be thought of as being there for the long haul, and I think 15 years definitely cuts it!
They are telling me for years to make a paid plan and that was always the idea. Maybe it's 15th birthday is good for that too.
With all due respect, you probably should plan around this. What if you suddenly die tomorrow? I'm not trying to threaten or scare you, its just that you (and your customers) should plan around this. An example of such planning which likely fits the glove for all readers here, is to have your private key or password at a notary.
It's ok, I did do this; like said above, the source (and all keys/access) are in escrow at a notary for the projects I run in case of bus-hitting. Also, I am not working alone, but yes, I will rethink this again as it was a while ago that this was arranged.
My (very brief, IANAL) intro to civil law in Switzerland covered an interesting detail in contracts: contracts (or parts) need to follow “good morals” to avoid being invalidated [0], an example of which would be that the contract shouldn’t bind the parties for perpetuity.
So this part of the contract couldn’t be enforced by the other party (the customer), while the provider actively advertised it. I wouldn’t be surprised if this would be seen as bad advertising and would lead to potential compensation for customers and competitors, if they stop providing the service.
[0]: https://www.admin.ch/opc/en/classified-compilation/19110009/...
Edit: added the English translation of the link.
Do we really care though? You can just scrap the company and do an other one.
An other option is just to change the terms, display a pop-up to users saying "Hey, our terms changed, do you accept the new ones?". You can bet 99.999% will click yes and not read the new terms.
In this case probably yes, that's what the limited liability is for in companies. Though if you have legal issues with the old company, some jurisdictions may not let you start another one (or at least be director).
If it's Google (Photos) or HP (lifetime printing) making the promise and driving competitors out of the market, I think it's a valid claim.
> An other option is just to change the terms
I'm not sure if that's an option if your customer has signed up under "lifetime" terms. While these parts of the terms they've signed up for are void, they could claim that they've had damages (costs or missed opportunities) by this "immoral" offer.