Well normally lifetime doesn't mean YOUR lifetimes but rather the life of the product. So as long as the product continues to be sold/supported, you get what is promised. Like this is real common on enterprise switches, they come with lifetime warranties. So as long as the switch is on sale, you can get a new one if it breaks. Once it goes off sale there's an EOL period of a couple years when you can still get support, then it is done, and if it breaks after that, you can't get a replacement. Usually works out to be 10-15 years in practice for these particular things.
Same deal with Sonar Platinum. As long as Cakewalk keeps making it, you get updates. However if they discontinue it, then it is done because the lifetime of the product is over. How long will that be with Platinum? Nobody knows, including Cakewalk I imagine. Generally with "lifetime support" type products it is done in part because the maker doesn't know when they will discontinue it, they have no specific date planned. They'll keep it current until it no longer makes sense to.
However, based on the age that people in affluent countries make it to and the general changing nature of technology, I'd guess your lifetime will exceed Platinum's lifetime. It would not surprise me if in 10-15 years things have changed significantly enough that Sonar morphs in to or is replaced by a new product and Platinum's life (and thus updates) end. However statistically speaking you should still be with us, and hopefully you will be :)
Either way it should be worth while. So long as its life and your life exceed 2 years, both extremely likely, it is cheaper than paying yearly maintenance.