I think this is due to the extent of the maintenance efforts by the team that develops the app, not due to the gems themselves. A monolithic app means that no code is legacy code: a conscious, gardening-like effort needs to be continuously done to keep interfaces standardised, modules documented, code clean, and critical components (like user management) top-notch with focused testing and code quality metrics. I've worked in different teams on 5+ year codebases that hold up to this day by being disciplined on this terms and never buckling down on complexity.

What I've also seen is that a team with plenty of churn or no time for maintenance will have exactly the problems you describe. As knowledgeable people leave the team, the old code becomes "legacy", new people come and partially refactor it, maybe adding some new gem and planning to migrate the entire codebase later on to it (which is very hard to make it happen). Stakeholders' pressure will force the team to focus on delivering features and only tackling debt when it gets to an unattainable state, and then is when consultants are called. I think this is why many freelancers suffer the worst of Rails (or any other framework) if they don't usually work with greenfield projects.