I do realize putting "lifetime" on anything brings heavy emotional weight. But in the end it's simply another contract which, even without any malintent, get broken all the time. If you are the damaged party you can use the legal system to seek damages, if you feel the inflicted damage warrants going through the trouble.
Besides FastMail is in Australia, my father is in the UK and I am in Japan ...
Also, would the court order FastMail to continue to offer service to my father, which is what I want. I very much doubt that. FastMail would just be allowed to make the offer of the $15 plus some half-price deal on another plan, which they have done already.
On the other hand, companies do seem to rather dislike bad publicity. So when they try and back-out of past agreements and contacts that no longer suit them, it is far more effective to call them out publicly on forums like this one.
Now, I don't know the details of the contract, but generally, the value this is about is not $15. The value is the damage caused by them failing to fulfill their contracts, which would be the discounted value of the market price of all future payments needed to replace their failure to perform.
There is no general legal option for one party to a contract to simply pull out and give you back your money. They owe you what they promised in the contract, and if they don't deliver, they are liable for the damage that that causes for you.
If this was about serious money, that's how my lawyer would argue it.
But for $15 I'm not going to try that in a small claims court in the UK say. I've been there once and the judge was one of the most condescending, pompous dimwits that I'd ever encountered.
Unfortunately, I have no idea whether you could make that claim in the UK, as you signed a document which sets Victoria, Australia as the venue for lawsuits. (And that there provides the reason to purchase services from a company that has an EU presence.)
Just wondering: Where did those $15 come from?