Yes, you can learn anything at a superficial level in a couple weeks. If you've spent the last ten years maintaining an ASP.NET WebForms app running on IIS, then within a couple of weeks, you can contribute code to a Go microservice running in Kubernetes and using Kafka.
And that kind of "basic competence, with the ability to learn more" is what hiring managers want out of a junior dev, so if you don't keep your skills up, well, great, you're as useful as a junior dev.
But what they want out of senior devs is people who know the ins and outs, who understand what to do and why to do it and can explain that to others, who can avoid the pitfalls of bad tech choices and misguided implementation patterns.
If you've got decades of experience on your resume, my guess is that you want to be evaluated as a senior dev. And if you're a senior dev who doesn't know modern tech, then... you're not that useful. Your encyclopedic knowledge of the WebForms event lifecycle doesn't give you any particular insights into the design challenges of a microservice ecosystem; your ability to tweak IIS for maximum performance won't help you write a Helm chart.
If you want to be hired as a senior dev, you need to understand how the modern world works, and all the posturing around "I could learn it in a week" or "I prefer to focus on problems" doesn't impress anyone.