Strongholds shouldn't be that affected - they can't drop below 25% [1], so 250,000 control points into Stronghold, which is a lot more net weekly Undermining than any individual system has got for a very long time discounting the recent data exploit (and even gross Undermining is almost always a lot less than that). Sure, you can't max them out even higher than that without constant effort but you mostly don't need to - a 25% Stronghold and a 100% Stronghold are equally invincible for now.
What it will mostly nullify is the accumulative effect of casual reinforcement of systems in Exploited/Fortified state. The vast majority of systems see <1000 CP of reinforcement a week, with most of the rest seeing <5000 CP, so any significant decay at all will keep those pinned very close to 25% of their band, and have the only (defensive) moves that matter be those made by larger organised groups who can rush a system by a full state in a week or two.
It'll be interesting to see how much practical difference that makes to the map since most successful reinforcements were already from the big groups, of course, and none of this makes it any easier as such for smaller groups to undermine or creates undermining options with comparable CP rates to the fast reinforcement ones, so I think this is only going to be buying Frontier a bit more time to fix the bigger balance issues. I think we'll still see R:U ratios in the 15-20:1 range in the short term.
[1] This of course is a threshold Frontier could reduce later. Move this to 1% so any unmaintained system is vulnerable to being knocked over by a last minute rush of 3-10k CP, and oversized powers suddenly get a bunch of opportunities to be cut down to size. Move this to -1% and there doesn't need to be workable player-led undermining at all, though it does change the nature of what players are defending against significantly.
