They aren't specifying the period for which they will be providing bug fixes and maintenance. Which means the software has an unknown (and judging by similar cases, probably very small) amount usable life left.
As opposed to how the app store paradigm of low-cost one-time purchases has lay people believe, software is radically different from most shelf-sold commodities they are used to buying in stores, in that it is destined to become less useful with time, to rot, if not cared for. Unmaintained software is prone to security vulnerabilities, gets harder to document and understand, and eventually becomes unusable on modern hardware. That "you're not losing your email client" is only true for today; having a working binary today doesn't guarantee you'll have a secure, working tool a few weeks/months down the line.
If the app store industry is intent on continuing to sell software as minimally critical to people's productivity as email software, they are going to have to come up with a persuasive, standardized, binding method of informing people of terms of support, and learn to stick to the terms they have set. App stores and their respective platforms are only a few years old; give it some time and people are going to collectively get much wiser than to tie their critical workflows and data to opportunistic pieces of technology built by bright young people whose top incentive is to make a fast buck and/or pose as "indies" to get employed by the status quo.