Since the decay doesn't go below 25%, existing systems are safe unless actively undermined. I think most systems are below the 25% limit anyway. But making the decay unlimited could lead to a domino effect of a collapse for smaller powers that don't have the manpower to constantly maintain everything. And be outright untenable for small player groups trying to maintain a few backwater systems the big groups don't care about.
Problem here is, to acquire new systems you first need to reinforce existing ones to at least Fortified state. That's the hard part for a small group or a single player to do as it was before current update, and decay will make it even harder. A system I would like to eventually acquire is juuust outside of the control radius required from a Fortified system. That means I either have to get an Exploited system to Stronghold or acquire and fortify an additional system in between. I guesstimated it to take approx. 1...2 years for a single person to do; and several months for a group smaller than 10. Add in the time to establish a redundant control chain for resilience to hypothetical undermining.
10...50 player-hours I think is a gross understatement, unless you luck out and have a system that has the highest merits per hour activities available (like mining--and demand for the mined valuable minerals at local stations*)
Now that is a really good point--why undermine a system and draw attention to it when you can sit quietly until it decays to 25% and then attack with overwhelming force? Instead of constant struggle you end up with "submarine warfare" where the adversaries sit very quietly still, and then launch the nuke
I think instead of control decay a more interesting mechanics would be random riots where a certain % of a power's systems go to an unrest state which would make firstly undermining easier, and secondly keep owning powers busy constantly putting out brushfires. That would not punish small player groups, but adds needed dynamics to the stalemate.
*I tried mining the common cheap minerals in a system--the merit/control point payout was not worth it. And the demand for Void Opals, LTD-s, Pt etc was non-existent (something like 10 tons per article).