I'm not even 30 and I feel like my skills are in decline. I can cover it up with experience - I know the libraries and tools and how to use them, and that can make you 100x faster. But I'm getting set in my ways after seeing what does and doesn't work so many times, and I'm losing the ability to just do something without thinking it through.
You are becoming more valuable (in the sense of being able to deliver more absolute value). But you are becoming less of a commodity, which means employers are also becoming less of a commodity for you.
To prepare for the future, you need to look into marketing yourself "wholesale" (that is, sell completed projects rather then into a team), and/or establish a network of people who care about fundamentals and not the latest hotness.
I turned down a hadoop job awhile ago, because it was hurting me to do things so stupidly. (I managed to do the same thing on C at 1/10 of the time on one machine as the project did on 10 machines using Hadoop, but they insisted that "hadoop is the thing"). If I was hungry for food, I might just deal with these pains. I'm obviously less employable right now than I was in my 20s when I was happy to use any stupid technology - but I'm happier for it.
IMO a more likely explanation is that because you have more experience, you are becoming more aware of your limitations--limitations that have been there all along, but you didn't notice them when you were younger.
> I'm losing the ability to just do something without thinking it through.
You still have that ability, but now you understand the risks of doing things that way, so you're reluctant to do so.
I agree with the other comments. A person with the ability to do something before thinking it through is called a "junior engineer". A senior engineer is someone who thinks it through. A distinguished engineer knows when it's not even worth thinking about it :-)
Where I worked the "10x" engineers (who are legendary) are among the eldest.