That's a TT Smart 750w Bronze. It's as bad as it gets in TT's lineup, and while having the right 4x 6+2pin pcie, it's really not designed for the stresses a high current gpu like the 3070 can demand.
To put it in car terms, it's a Crown Vic, not a Mustang. Both might have the same 8cyl 302, but one is designed for grandpa and his driving habits, the other for the grandson and his driving habits.
Capacitors are the major component electrically inside a psu. If you think of a capacitor in terms of a florescent light bulb, they are extremely similar. The more use it sees, the faster the end turns purple and eventually quits. But if the light is not used, it'll last for years and never turn purple. You have 3 years on an office psu. No worries. Doesn't really matter how long it sat on the shelf. That's not what makes those caps go bad.
It's a serviceable psu that'd go very well in a normal use, office type pc, but won't handle the instant high stop/go/stop/go that a gaming gpu can demand. It'll last a while, but how long generally depends on the loads the gpu is seeing. Could be a day, could be a year, could be more.
Older gpus used a 6pin or 6+2pin or combination depending on actual power draw. The 30 series saw the release of a 12pin to take the place of a 6+2 or 6+6 or 6+8 or 8+8 design, which is why they supplied the adapter. That adapter matches up with that gpu, no other. Having pins missing wires just means it's a power limited card, and doesn't require more pins populated, but still uses the same 12pin junction. It's a money saver, all the 30 series use the same connectors instead of using different styles for 6, 6+2, 8+6 etc.
It's technically possible to provide upto 720w with 4x 8pin pcie plugged into a 4x8 to 12pin adapter. Pcie is restricted to 60w per pin, the adapter and gpu isn't. The 3070 FE is @ 220w so won't require every power pin populated in that 12pin.