I don't see how this would give teams an extra year of control. Rule 5 draft rules would still apply and option years.
40-man spots are so valuable it's hard to imagine someone surviving long enough to reach 8 years without having been a Rule 5 pick, run out of option years, or DFA'd to make room on the 40-man.
If you aren't on the 40-man after 7 years they would probably welcome the 8th year lol
Unless I'm misunderstanding what this means or missing some other scenario.