From what I've seen, at the bare minimum, it will break down completely back into plant polymers faster than other plastics could hope to break down into anything non-dangerous to the environment, and even if it does break down into microplastics quicker, I'd rather have something like that, which can then later break down into plant polymers, rather than something that slowly leeches microplastics into the environment for the next few centuries, and doesn't really break down into anything much less dangerous past that point.

To cite some interesting points from the paper you referenced:

The biodegradation of polylactic acid occurs in two main steps: fragmentation and mineralization. [...] which can be biotic or abiotic. For instance, biotic hydrolysis involves microorganisms and/or enzymes, whereas abiotic hydrolysis involves mechanical weathering.

This means it can break down via multiple mechanisms, with or without the presence of any microbes, but only given specific environmental circumstances, which is why it doesn't work well in aquatic environments, as previously mentioned. However, some of it does still break down there, and if it later exits that aquatic environment, other processes can begin to break down what remains.

The authors concluded that polylactic acid and its blends are similar to non-biodegradable plastics in terms of biodegradation in aquatic environment.

[They] proposed that low temperatures along with low bacterial density make the sea water unsuitable for the biodegradation of polylactic acid.

However, on the microplastics point, while they do state it degrades quickly, in terms of overall quantity of microplastics produced, it's actually lower than other common plastics.

The authors reported that polylactic acid forms almost 18 times fewer microplastics as compared to the petroleum-based plastic, polypropylene.

They do still mention that it will still likely have many negative effects on marine life, though, even given that. Surely we'll stop dumping plastics in the ocean now, for the good of the planet! Or not, because profits matter more, am I right?

From another study, it seems that soil with certain combinations of bacteria, at regular temperatures found in nature, could mineralize about 24% of PLA in 150 days, which is pretty damn good compared to how long it would take non-bioplastics to do so.

And of course, when put into dedicated composting facilities that can reach high temperatures, PLA can be composted extremely effectively. And this is just regular PLA we're talking about, not things like cPLA, which can be 100% composted within regular composting facilities within 2-4 months. (coincidentally, most biodegradable utensils are now made of cPLA)

I wouldn't doubt we start seeing even more compostable variants of filament for 3D printers specifically popping up as actual distribution and manufacturing for the material becomes more cost effective and widespread. I was able to find cPLA filament at a reasonable price just from a simple search, and there's even a biodegradable flexible filament as an alternative to TPU, made of oyster powder, which is 100% compostable (though is about 4-8X the price of regular TPU per gram as of now)

None of this discounts any of the current environmental impacts of 3D printing materials, of course, but a lot of PLA now can already be almost entirely, if not actually entirely composted in local municipal composting facilities, and there's even more compostable alternatives that exist today.

I compost my failed or no-longer-needed PLA prints, and my city even explicitly states to put it in my compost bin, as it's supported by our composting system.