I don't think it's a disdain for .Net. It's more of a disdain for ASP.Net Webforms. I'm definitely not a youngster (I'm 41). But he's been doing Asp.Net Webforms for 12 years. He hasn't shown a "knack for constant improvement". He hasn't been keeping up with the frameworks and technologies that MS has been pushing - Asp.Net MVC, WebApi, Entity Framework, etc.
If he has been doing WebForms instead of MVC, he's probably not up on all of the basic table stakes client side stuff like BootStrap and JQuery.
I know at 41 years old, if I want to stay in development and not go into management and command the salary I want, I can't be complacent. The minute that my company stagnates, I must find another job. That means for me, being a full stack .Net developer:
1. Web - Angular, JQuery, CSS, TypeScript, and Bootstrap
2. Server side web - Asp.Net MVC, WebApi, WCF
3. Knowing how to speak the language of an architect (DDD, Design Patterns,everything that Martin Fowler writes)
4. Database theory and maintenance and EF.
5. Testing - front end and back end automation testing.
I'm not bragging, I know lots of developers who can tick off these checkboxes. If you're not willing to aggressive learn, this isn't the field for you.
I just wanted to add that I do have professional WebApi, Angular and JQuery work on my resume, but the big thing that seems to be the blocking point is MVC. I don't code traditional webforms development anymore (I don't use controls anymore, it's more JS calling json services), but there's no resume-friendly name for it because I'm not using a specific framework, and I'm guessing that to an HR person, all that means is "not MVC".
It's a crap-shoot on which technology is going to take off. Microsoft was pushing heavily that MVC was just another way to do things during the events I went to. I tried to push the boundaries of what my company let me work in, and that just happened to not be where the market went.
In hindsight, keeping my eye open on the job market is something I should have done more.
Another honest assessment: if MVC is the only thing blocking you from various better positions, just say you've got it on your resume. Then spend a couple days after work going through MVC tutorials. "New Project->Web->MVC->OK. F5." Bam, you've got MVC experience.
Seriously it's way easier than webforms so if you've got that much webforms experience, you'll have absolutely no problem picking it up. Within a week you'll know 95% of what you'd need to know on most tech interviews and you'll do fine at your new job. Stuff varies so much company-to-company that MVC will not even be one of the top five differences.
In my previous response I was thinking more you were looking for some big change into something very cutting-edge. That would be a more difficult proposition. But if you're just looking to go from webforms to MVC, or even to a Java-based infrastructure, it's not a big hurdle. Like others have said, the HR filter will be the biggest problem. There will be an age bias too, you'll just have to deal with that.
FWIW I don't think you should expect a paycut or a cut in "rank" either just because of changing technologies. You'll get caught up on the technical side very quickly. Outside of extremely technical companies, "rank" is far more about knowing how to get through a release cycle coordinate teams than it is about specific technical knowledge.
All that said, just be warned, working on websites in MVC (or really any technology) is really no more interesting than Webforms. (In fact webforms may be more interesting because you get to invent your own way around its inadequacies).
If he has been doing WebForms instead of MVC, he's probably not up on all of the basic table stakes client side stuff like BootStrap and JQuery.
I know at 41 years old, if I want to stay in development and not go into management and command the salary I want, I can't be complacent. The minute that my company stagnates, I must find another job. That means for me, being a full stack .Net developer:
1. Web - Angular, JQuery, CSS, TypeScript, and Bootstrap 2. Server side web - Asp.Net MVC, WebApi, WCF 3. Knowing how to speak the language of an architect (DDD, Design Patterns,everything that Martin Fowler writes) 4. Database theory and maintenance and EF. 5. Testing - front end and back end automation testing.
I'm not bragging, I know lots of developers who can tick off these checkboxes. If you're not willing to aggressive learn, this isn't the field for you.