Well you could be doing something wrong so why even put it on a dash cam.
You mentioned Street View and someone could try and guess at your speed based on landmarks in a video and distance / time. For me personally I'd rather just have it recorded versus being open to interpretation.
I was in an accident where somebody pulled a u-turn right in front of me, I T'd them, and had it all on cam (true story). Their only defense could have been that I was speeding. I wasn't (from what I can tell) and it didn't come down to that but since then I've always opted to have that on the GPS also.
I didn't mention interpreting speed from video. You can't. Unless your speed is obviously dangerous, no one can prove anything. Even then if driving dangerously, it can't prove your speed, only show dangerous driving. Hopefully, no-one would record themselves driving dangerously or be stupid enough to drive dangerously in the 1st place.
I mention Street View not for calculation but rather because it provides pictures confirming the location and views of junctions etc from multiple angles including the air. A far better view than a dashcam alone.
GPS's real purpose is to prove speed. The trouble with GPS is eg. if travelling in a US residential area where the limit is 25mph (I know if varies by state), and GPS shows you over 25mph, then the footage of the accident becomes useless at proving you weren't at fault. In your case, the other driver in your example, having turned across you is definitely at fault in the majority. You say you weren't speeding from what "I could tell" and no doubt your dashcam proved your weren't at fault in respect of the position of vehicles / manoeuvres. However, GPS may have proved otherwise on the speed.
If it does, being proven over the limit, even by only a small margin you'd accidentally creeped over without noticing, has the potential to introduce arguments of contributory negligence ie if you were travelling those few mph slower, the accident could be avoided. This takes you from a position of 100% blame on the other driver to some percentage of blame on you. That is enough to potentially affect your future insurance premiums as your insurance company potentially has to foot a percentage of the bill. It may also lead to prosecution if the police in your state have the power to seize the GPS embedded footage. Without GPS, as you already found with "so far as I can tell", it's impossible from video to tell whether someone is travelling at 25 or 30mph leaving the blame squarely 100% on the other driver and you in the clear.
Opinions will vary and I've seen posts like mine and posts in support of GPS. It's personal. For me, I'd rather than have something that simply showed what happened rather than potentially hangs me as well. When choosing a cam, there are plenty out there that don't have GPS or have it as an optional module only. So just be aware there is choice and pro's and con's to both.