If you were paying attention, immediately after the Orthrus payout reduction, the anti-xeno community shifted focus from spires and sampling to alert systems and, ironically, NHSS 4 with Orthuses. This caused rapid proliferation of alert systems before they could evolve into invasions.
You're correct that one came after the other, but I don't think your conclusion that this move (or the resulting complete shutdown of invasions) was
because it had the higher payout is necessarily correct. A substantial amount of the military effectiveness - if not necessarily player count or activity count - comes from the organised groups who aren't primarily in it for the money.
An important point of the May rebalance is that Frontier also made numerous rebalances to the war effectiveness
per action of every single action type in the game, with the collective aim of making the Thargoids considerably easier to contain and defeat, completely independent of their credit-earning potential. This predictably shut down invasions, yes, just like it did the last time they made the Thargoids easier to contain. (The last time it happened, the main cause was carefully-targeted sampling, which isn't at all an efficient credits earner)
On the other hand, heavy spire+sampling attacks continued (less profitably in the case of the spires, but still intensely!) to destroy Indra's control sphere over the following weeks. Sure, some profit-motivated players may have stopped [1], but war-motivated players were very definitely carrying on. And similarly, many of the major organised groups currently doing a lot to shut down populated alerts and invasions have been doing that consistently throughout the war regardless of payout - it's just more effective now.
[1] Given the change to make spires stop regenerating, they'd have to eventually anyway.
Then, last Friday as I am sure you recall, we got a surprise update which was intended to "fix" the previous update so that Thargoids could skip alerts and go straight into invasions in human populated systems.
Sure - lots of players like fighting in invasions, because it's one of the easiest ways to contribute to the war by combat, so bringing some back even if the Thargoids are losing ground makes a lot of sense (and has been a player suggestion for about a year...)
Given Paul's comment at
https://forums.frontier.co.uk/threa...r-changes-6th-june-2024.625949/#post-10385969 ... that particular rule change may have been a surprise to some players, but it looks like Frontier had been planning it as a "when Indra becomes vulnerable" move for at least a month.
Also as part of that update: they made
Control systems have some invasion-like combat content. If that goes well - starting tomorrow around Thor, in theory - it might draw off some of the combat pressure on the very limited number of invasions and leave more for haulers/passenger carriers to do in those systems.
Of course, in the slightly longer term, spires, invasions, surface attacks, etc. are all apparently on the way out - five Titans down, three to go, they're going to need to pull a fair few more well-telegraphed "surprises" before the "end" to keep any of them going more than a few months longer.