That said, if that's the central issue definitely a lot of the points raised are overblown.
I don't really see them as straying too far from their desired focus. Things and technologies change at big companies and developers need to have some flexibility. Learning basics of ERB templating isn't a big ask and it's conceptually similar to all sorts of different templating engines.
It also sounds like more front projects were coming and things were going to get better. 5 months is nothing in the grand scheme of things and it's an awfully quick bail. Especially considering the quality of Shopify as a company. If the FAANGs are the Kings of the programming world, Shopify is a Duke or Baron. Eyebrow raising to leave a company like that unless they ended up in a clearly better spot.
On the one side: I get it. As a developer you want to - and the industry expect you to - keep learning, always be on the increase, etc.
But on the other hand, when you think about it, once the main architecture is put down and the main issues are solved, most software engineering jobs are just repeating the same trick over and over again, copy / paste / edit some names, done.
The last part is where you want to end up at as a company, basically; build features end to end in a reasonable time. But it's not what most developers want, they want new challenges, they want to stay in the problem solving domain.
I think as a developer you need to accept that a big chunk of your job, and what your employer wants you to do, is kinda boring. It's not solving new problems, it's extending the functionality of an application using existing solutions.
And if you're hired as a front-end developer or whatever, the hiring company may not want you for what you can become, but what you already are.
And honestly, just trying to find decent developers that don't need to learn your stack is already difficult enough.
I don't know. I think it gets less and less true as you get away from web development. The fundamentals stay the same but how they intersect with what you are doing can vary widely. If you want to, you can encounter domain knowledge intersecting with the problems you are trying to solve, different architectures for complexe systems coming with their own set of constraints, different project structures, different ways to deal with end users requirements.
I think they are plenty of opportunities to do boring things and plenty to do interesting things.
It strikes me as a misunderstanding on how career evolution works in these companies (and, I believe, most tech companies). A senior eng will focus less on the pure technical aspects (language, framework, etc), but rather will have the soft skills allowing them to carry larger projects/impact.
For instance, with experience, a senior can identify design mistakes early, can weight the value of different solutions based on aspects like time/eng-cost, tech debt, complexity, risk.
In a company of the size of Shopify, shifting away from "Rails ERB templates" to a more modern solution definitely require these skills (and also probably way more than 6 months of tenure). They could have proposed this project, tried to get some buy-in and learn new "frontend skills".
I am finding this whole thread very weird!