I agree 100%
Why do people buy themes ? Because they look good.
Gutenberg provides better design features, specific block design for a unique and coherent theme design. More freedom and power in the hands of the theme designer, in short.

I am always puzzled by recurrent comments from web designers regarding the danger of customers messing with their websites. Here's my take on this issue.

Gutenberg answers the opposite needs of the DIY customer and web designers quite well.

If you're a DIY person, it's more fun than ever. Total freedom with Gutenberg.

If you're creating a web site for a customer - in essence not a DIY person, keep that in mind -, no worry : you can lock blocks into place, can't you ? There's a solution for that : template_lock, built in core. Want to lock block-styling options as well ? The use of ACF blocks or any equivalent solution is a simple answer : data is decoupled from the design. Appropriate freedom for the customer.

Yes, it is easy to turn a great looking theme/website into something nondescript. All you need is an access to the website's backend and no knowledge of website design. The typical scenario is the small business owner hoping to save money going the DIY route, buying an option packed premium theme believing he will get the same result as the demo that seduced him. What ensues is usually a total design disaster. But all is fine : he did not want to invest in the first place.

Still, some web designers seem to give admin rights to their customers. Why ?
Worse : many use such best-selling DIY premium themes. These were not built for web designers. Others give access to their favourite page builder, with some quick instructions. Don't complain if mayhem follows. Don't be lazy.

The same goes for standard Gutenberg blocks, conceived with a DIY approach in mind to compete with Wix and the likes. Such a crucial strategic choice required no barrier between data and design. For that reason, unless you love spending time on customer support, avoid them if you're a web designer : they were not meant for you in the first place. (But they are awesome for theme designers, selling good looking DIY themes.)
Again, do bear that in mind : DIY freedom for the customer is the enemy of any web designer.

As is, you can create and lock block templates (populating a page, a post, with a set of predefined blocks, locked into place ), but you can't lock block-styling options easily, not yet (as far as I know). No need to complain, Gutenberg has you covered : you are free to create as many blocks as you want, with full control. All the tools you need to do so are available. Some block building tools and block libraries already offer such options out of the box. They do come with a price, as they are built for professional web designers.

With such tools, securing access to parts of a WordPress website, guaranteeing the integrity of its design, while enhancing its flexibility, has never been so easy.

True, the era of spaghetti code is now gone, no more messing around using snippets of php, tons of plugins, shortcodes and stuff to tweak a template. WordPress embarked on a much needed journey, reinventing itself as a clean and reliable solution for the future, with higher and stricter standards as far as code is concerned. That leaves out many "intermediate level" web designers who relied on such cheats/tricks, unless they adapt. But is it bad ?

In any case, I have never seen customers asking for admin rights and full control of their website. They are customers because they want to trust the expertise of an agency and not waste time. Appropriate user roles are enough, if they need any - making regular content updates on specific pages, being able to blog, are legitimate needs, accessing everything is not. And again, in such cases, lock what is supposed to be locked.
And yes it is easier and faster to set up and fine-tune with Gutenberg.

Bear in mind that most of the time customers don't want to handle content updates themselves, and don't even want to run a blog. Just don't ever sell them how easy it is to do so with WordPress, snapping your fingers, because it is not, unless you love to provide endless customer support on trivial stuff. Spare some precious time, yours and theirs, and provide updates/upgrades for a fee. That works with content too. Lock everything that should be. And make more money in the process. Reward yourself. Your customers will love you.

In short the issue is often a business model issue : a misconception of the customer's needs coupled with a loose sales strategy (or none at all), on top of a lazy choice of tools. For a web designer, selling the DIY mantra to customers is a very bad idea. Giving in to a full access prerequisite is also a no-go. If a would-be customer requires it, just don't take that job, leave hell to others.

Even if you did - giving the customer full access to the website - and a nasty mistake happened, a one-click backup restore would save the day, would it not?
Undesirable behaviours are best avoided through a proper locking strategy, though. Educating the customer is key. So is adopting a proper approach to building a website for a customer : use the right tools.