The manual thumb safety on the Shield is not a mechanical problem/issue, it's a training issue.
While I haven't carried a 1911 in 10 years, I started out on 1911's. And while my Shield is the only gun in my current carry rotation with a manual thumb safety, I choose not to use it.
However, in two days I will have owned my Shield exactly one month and I am already in the habit of sweeping the safety off, even though I never engage it. As a matter of fact, I am now in the habit of sweeping the non-existent safety off on all the guns I carry.
It's a matter of training to sweep the safety off. After literally hundreds of times of practicing the draw and sweeping the safety off as the gun comes up, it's merely a matter of getting it ingrained into your muscle memory.
I never had an issue of forgetting to sweep the safety off on a 1911 when I carried them, and to date I've never had an issue of going through the motion of sweeping the unengaged safety off on the shield. And it has transferred over to the XD9, the LCP and the full size M&P, none of which has a manual thumb safety.
Again, it's nothing more than a training issue. It's not a mechanical issue.