The thing is that at the time of now, PHP 7.0 isn't old and unsupported. For Debian users, will be new at the time when Debian 9.0 (Stretch) will be released. It will be supported until ~June 2020 (~June 2022 for LTS). See next point...
It won't expire before ~June 2022 (same like TYPO3 8 ELTS) for Debian Stretch users.
But the thing is: TYPO3 has a 1.5 year release cycle, while Debian has 2 years and RHEL has 3-4 years.
That means, if you want to start a project in May 2017 on one of these platforms, then you will need to to use a TYPO3 version (7.6 LTS) which is not the latest version and will be EOL in 3.5 years (with ELTS, otherwise it's just 1.5 years).
Even worse: If RHEL is a must (see the example that Lukas @rtp-lru reported) then he would need to launch with TYPO3 6.2 because RHEL7 only ships with PHP 5.4 (according to DistroWatch).