That isn't really an infinite amount of goods, particularly the beasts. With breeding pops they sell us a supply of them, and then we're in charge of producing them at our own cost. It's not that realistic to expect we're going to need the breeding supply replaced. Teaching how to use them is a one time thing as well unless such a large portion of our society is destroyed that records are lost.
Their weapons are adamantine so they're probably recoverable from our dead and won't need replacement anyway. Meanwhile we're providing them a steady supply of weapons whether we're getting none or a thousand from them. Also their weapons are probably not that impressive anyways because if they were then they wouldn't be interested in Tau weapons. Over time they're probably going to be phasing out their current weapons for gradually enhanced Tau weapons, at which point they'll just be paying us in the surplus gear they phased out for what we're paying them.
Given that we're transplanting them into a new enviroment alone I'd say we're going to need quite a few tries to get a proper breeding population up, to say nothing of the fact there are hundreds of uses to the bees alone and dozens of skills that need to be learned.
And of course the new Legio Helheim is going to be taking casulties we're going to want to replace as well.
So? They're impossibly tough, but lower level relics do get destroyed, especially since we're likely going to be against alpha psykers with them, and not mere thousands tens of thousands even hundreds of thousands if we want to go for them.
They're weapons are incredibly impressive, they're relics warp enhanced nonsense that astartes chapters wish they could have, but only the oldest are lucky to have more than a handful, they're meant to be impossibly rare and the Saurus are handing them out like literal candy, the fact we're so eager to buy them should be indicative of how valuable they are.
No it means they want additional options, they don't have guns so they want guns to provide new ranged options for example. After all, it doesn't matter what level of tech weapon they have, they're equally deadly in the hands of the Saurus because of their abilities. Could be a literal rock give it enough time and it'd destroy power swords.
In other words they're not going to be phasing out their current weapons, they're just supplimenting their arsenal.
Their weapons improve over time via psykery, but I suspect the base is part of that equation. Therefore, an extra +10 or +20 to their basic, brand-new stuff is worthwhile to them.
I believe Wombat noted that 200 years from now we'll be buying that stuff back as relics.
Relics, so not really.
For them the basic isn't going to change the effectiveness of their weapons after a point, its just to give them addititional options.
But, yeah they're also a long term investment, since we can buy back super powered pulse rifles later on.
