Well, I think it's more complicated than that. They just never get the chance to test under full server lose until they go live with an update.
Ok, but that just means that they've been suprised 8 times in a row, that's all. Fool me once, shame on you, fool me 8 times? Come on. Gotta be a way to simulate this better, or give better testing rewards (and advertise them) to get people to help. Or if it's just server capacity, have resources they can bring online for a couple weeks until the surge dies down. Every time they add a new season/patch, we get huge queues, server crashes, and other hiccups that aren't related to the 'bugs' in the gameplay they missed. How many times can they act shocked?
For all the help they get with testing, doesn't seem like they implement a lot. Did they do testing rewards this time around? Didn't see anything? That's more about the bugs/design than the server loads, though.
And then, gotta guide the testing somewhat. Kinda seems like the ground aspects didn't get nearly the testing the space side did. Mostly because they suck at ground, and don't recognize that they need to make ground rewards bigger to justify the harder missions and longer times to complete.
Think about it: when's the last time you did a ground STF? Can you even find a PUG group to do one anymore? Last couple times I tried, I couldn't get one to start. Can almost never even scrape one up in the EliteSTF channel anymore. Rewards are all the same, so you do the easy 5 minute space one instead of the harder 30 minute ground one...
They've said they always suffer revenue loss during the tribble test weekend. So it becomes a tug of war between the the need to have revenue vs the need to do proper load testing (which can only happen with real players and not some scripted/recorded player actions). This time round, they chose not to have a test weekend, which kinda implies they cannot do a full capacity load test.
They can't just temporarily add extra servers to tide over the initial surge, it costs money. And what are they going to do with the extra servers after the surge is over?
As for ground adventures, I actually raised the point that ground rewards simply are not as good as space rewards in the tribble forums. Likely other players also raised similar points. As a result, they've doubled the vendor purchase price of all ground gear at the last minute. Yes, its still not enough but at least they're doing something. Hopefully when they have the time, they can do a proper review and make more changes to make ground rewards even more attractive.