You should remove this consultant from touching any of your system. A consultant suggesting a complete rewrite of an application in a different language are going to cause a lot of damage to the system and to your company.
Consultants often make these kind of expensive, vague suggestions; the real reason they are suggesting to rewrite your application in their preferred pet language is to lock you in into their service, not anything for your own benefit. Find a different consultant that fits into your system, not a consultant that wants to retrofit your system into their world.
Most web applications won't scale better when rewritten in a different language, because the majority of web applications spend their time in database or API calls, which are already written in fast languages. You'll get better miles for your effort by optimising the uses of those database calls than by rewriting the same logic in a different language. And even when there's a good reason for rewriting a slow calculation in a different language, you usually only want to rewrite a very small part of the performance sensitive component in a faster language, while keeping the bulk of your application in easier to use scripting language.
When it comes to scalability, there are many different metrics.
In the vast majority of cases, the speed of the language matters none to scalability, designing good system architecture is where you need to design the system to scale.
making your application stateless, so your application can be scaled out to a cluster rather than a single machine
rearchitecting parts of the system that can run as edge service (close to the user) or as asynchronous tasks (not tied to request-response cycle)
modularity so multiple developers can work concurrently on the code base
writing tests so that anyone can confidently deploy their changes and not fear breaking anything
API designs that suits the use case
database design, as well as using multiple databases/data store that's appropriate for your use case
etc
These are all scalability improvements that will have much better return on investment than rewriting an application in a different language.
And in most cases when you want to rewrite a performance sensitive parts in "faster" language, the language you should be using isn't going to be .NET, but usually a systems programming language like C/C++ or Rust, which you can then FFI call from your existing application. .NET is too much still a high level framework that won't actually gives you the scalability or performance benefit of a rewrite.