Maybe it's just me, but 'adoption' sounds like a purely voluntary action. Don't mean to sound negative, but is there a word with less negative connotation than 'infection'?
They could also choose to run comparable software that does not require ios8 and not upgrade at all.
It's not about Apple being evil, it's the nature of computer security today. However they could backport security fixes, if they wanted to leave a period of time where people have a meaningful choice between freshness and stability.
So even patching is not forcing anyone.
There's no iOS 7.1.3 that would fix any of the many vulnerabilities fixed in iOS 8: http://support.apple.com/kb/HT6441 , and it's likely there won't ever be.
For example, CVE-2014-4377 ("Opening a maliciously crafted PDF file may lead to an unexpected application termination or arbitrary code execution") or any of the WebKit vulnerabilities are a pretty big deal.