> something less complex than life (but still possibly quite complex) which produces phosphine is also possible?

Of course it is possible. Everything is possible. But we have never seen such a process and we have seen life. So if life explains it but nothing else does except "possibly something" then life is the simplest explanation. The alternative really is no explanation, only speculation.

We could be living in a simulated universe, everything is possible, the simulation can include logically contradictory simulated laws of physics in other words "miracles". But such speculations don't really explain anything, they only say it is always possible that some other explanation may be found at a later time. Until an alternative equally good or better explanation is found it is best to use as working hypothesis the simplest explanation which explains all the observations, which seems to be "life" according to these scientists.

Yes, "life" is a "pathway". The link for the paper didn't work for me, but I'm thinking the issue here is the "pathway" with "life" involved is now the simplest system that can produce the observed results.

From the Quora article:

"The volume of Phosphine observed was stunning. They ruled out minerals blown into the atmosphere, volcanism, lightning and other known sources as there is simply no process that could maintain this abundance. So much Phosphine was observed we could conclude that the atmosphere of Venus is “teeming with life”."

One wonders if that isn't proving too much; if the atmosphere of Venus is "teeming with life" one would not expect the only such signature to be an excess of phosphine only noticed after centuries of observation.

I mean, I realize not all life necessarily engages in photosynthesis that emits highly reactive O2 into their atmosphere and radically and highly visibly transforms the entire atmosphere, but since the only energy source on the geological time frames those bacteria are going to be able to count on in the high atmosphere is going to be sunlight one would rather expect "teeming with life" to have very obvious spectra effects, i.e., "turning the planet funny colors" as life uses light energy in some sort of photosynthesis-like reaction, not merely creating an unexpected abundance of phosphine.

"Life scraping by" is a lot easier to believe than "teeming with life". "Teeming" life has no compelling evolutionary reason to also be well-hidden from us, and a lot of pathways to being very obvious as it exploits the energy available.

After some more thought, it's also just plain hard to believe in a pure atmospheric bacterium. I'm not sure we even have such a thing on planet Earth. Plenty that survive in the atmosphere better than expected, may even actively use it to spread, but nothing that lives up there. (Links to corrections solicited.) We don't routinely experience green days because of a mass of atmospheric bacteria blowing in above us and blocking the sun as they photosynthesize.

Energy it has in abundance, but where is it getting the atoms it's made out of? Carbon and oxygen are readily available, but you need more than that. Water is minimal in the atmosphere, and mere presence isn't enough, you need to be able to gather it against the strong osmotic pressures pulling it back out. (Earth life seems to have needed hundreds of millions of years just to grow on Earth land, a wildly more friendly environment than an atmosphere that still has puddles of nearly pure water to work with. Even a cloud is, by comparison, water-poor.) How do you get any nitrogen? Lightning may fixate it as it does on Earth, but then it immediately dissipates. It's very difficult to use N2 directly, although some Earth life manages. After that what's left in the atmosphere is some vicious acids and some noble gasses. So in the basic CHON equation, C & O are covered, N is exceedingly chemically difficult, and H is almost missing. Trace elements are also not available because they don't float, except sulphur (we use this ourselves, but it's not the most useful one), and while bacteria may get by without using as much of them as we do, I'm not sure you can make a life form out of nothing but CHON, let alone CO(Nish).

My bet is this will turn out to be one or more of the basic abiotic phosphine processes accumulating over long, long time because whatever processes that would consume phosphine are for some reason suppressed (e.g. lack of oxidants?).