Hi Gavin
I'm wary of making any cast-iron promises that I'm not in a position to keep, but let me share some thoughts here as to why I think you can continue to use Layouts with some confidence.
The key point here is why are we moving away from Layouts, which is explained by why we are switching towards the Block Editor (Gutenberg) for designing templates.
Toolset had the ability to make templates using HTML, but people like using page builders, and hence Layouts was born. But people vote with their feet, and many clients liked using Divi, Visual Composer, Avada Fusion Builder, Elementor, Oxygen etc. etc.
Adding and maintaining Toolset support for these page builders has consumed thousands of development hours. Although like many people we were initially somewhat sceptical about Gutenberg, we came to realise that as the project matures and with enhancements that we and others develop, you will be adding a pretty good page builder right into WordPress core.
Which means the rationale for maintaining support for lots of different page builders is eroded, and we can focus our efforts and resources on making the block editing experience a first class citizen when it comes to designing templates, and putting more time and energy into making Toolset a better product.
The expectation is that Layouts, too, would be eventually obsoleted to some extent by these developments.
Now, this isn't an attempt to foist what we want onto our users, it's an effort to deliver what our users need—albeit with some nudging—in a way that works for both them and us.
And if there are users for whom Layouts is a better solution than the block editor, then we will naturally need to maintain Layouts so that it can continue to be used as an alternative to blocks. Which still leaves us in a far better position than the current situation of supporting a multitude of page builders.
(As an aside, it's also a good example of the benefits of subscription-based pricing vs. lifetime accounts, because it means we have to keep existing clients happy and not just focus on new ones.)
So, either the Block editor will evolve in such a way as to make Layouts redundant, which is the one scenario where I see Layouts actually being retired rather than staying in maintenance mode, or each will continue to offer something unique that different segments of our user base need, in which case we have all the justification we need to look after Layouts and its users.
You are a long-time user so are actually using Layouts in a way that most current users are not, namely with an integrated theme that allows you to design the entire page, from header to footer, for which there are very limited alternative solutions out there, and which is a strong argument for the continued existence of Layouts. The Gutenberg project has a long-term ambition to also be used for parts of the page other than the content, but that's still quite some way off.
