Depends on who did your fitting and where. If you went to a qualified fitter, and he suggests the lighter shaft and you're satisfied with the result, why would you question it?
On the other hand if you went to a big box store fitter and hit on a simulator, you weren't properly fit.
If you're worried about the durability of the lighter shaft with your swing speed, you worry too much.
Clubhead and ball speed only means it makes a louder whack if you're still bouncing off the trees in the rough.
If you went to a qualified fitter and hit outdoors where you can see what the ball actually does in the real world, you and the fitter could determine what shaft gives you the desired result. Then you won't worry about it.

Good question to get back to your fitter. If it felt good at the fitting, then it should be fine..

Maybe the 74 g was too heavy?? Seriously, in the last 8 years since the 915 was introduced, shaft improvements allow lighter weights to be as strong/equal stiffness as prior heavier shafts. It’s not just vendor hype to upgrade every 2-3 releases to benefit from newer technology.

If you were properly fitted by a certified Titleist fitter, you should not have anything to worry about. How well did you hit the club?

Hey,
We’re did you get fit? If your club speed is 104 I would have some concern with the profile of the Tensei blue unless it was a X-stiff or TX. Just my thought without actually seeing your swing or Trackman data.

how did it feel on fitting? i personally struggled with lighter weighted shaft but do get better numbers with them however dispersion was more all over the place. did you try any othr shafts in the fitting?

I had almost the identical CHS and ball speed ( 103-105 and 155-160) and was fit into the Tensei 1K black 65S. I know there are a ton of variables that go into a shaft fit but just for reference that is what I was fit into.

If you had great results, I would not worry about the shaft weight at all. A lighter shaft means a lighter total club weight. We can swing lighter things faster than heavier ones. Enjoy the gains

The new Tensei Blue 55 is either 58 or 59 grams depending on if it’s R or S. With your 1.44 smash factor the torque of the S should be low enough that you shouldn’t be spraying it that much on your bad days.

As long as your not having issues with dispersion, lighter weight and softer flex is good for distance. There’s a reason Berkshire competes with what’s essentially a R stiff shaft, but plays with a TX. If you’re not having an issue finding the center and squaring the face for a repeatable result with a 58g R there’s no need to stuff a 90g TX shaft in your driver, unless you like that feel.

At the end of the day it’s really a preference thing. I don’t like the cumbersome feel of a heavy shaft so I go as light as possible without losing consistency. I have a 112 driver swing speed and game a Hzdrus Black G4 60 6.0. Most of the guys I play with think I’m nuts for gaming a 60g stiff shaft with my speed, but they also have me hit anchor in scrambles.

For a little more reference Cam Champ and his 124 swing speed plays a 68g shaft. Rory plays a 65g shaft with his speed over 120.