In my experience (of primarily web dev), this is not true, and the reasons it is not true are not limited to software architecture conflicts like you describe (although they happen too). Instead the problems I usually encounter are that:

* once you have shipped something and users are relying on it, it limits the decisions you are allowed to make about what features the system should have. You may regret implementing feature X because it precludes more valuable features Y and Z, but now that X is there, the cost of ripping it out is very high due to the backlash it will cause.

* once you have shipped an application, most of the time when you add new features you are probably slightly changing at least some UI, and so you need to think about how that's going to confuse experienced users and how to address that in a way you wouldn't have to when implementing something de novo. For an internal LOB app, that might mean creating announcements and demos and internal trainings that wouldn't be necessary for greenfield work.

* the majority of professional web dev involves systems with databases, and adding features frequently involves database migrations, and sometimes figuring out how to implement those database migrations without losing data or causing downtime is difficult and complicated.

* as web applications grow their userbase, the scale of the business often introduces new problems with software performance, with viability of analysing business-relevant data from the system, or with moderation or customer support tasks associated with the system, and these problems often demand new features to keep the broader business surrounding the software afloat that weren't needed at launch.

* software that has actually launched and become embedded in existing business processes inherently tends to have many more stakeholders in the business that care about it than pre-launch software, and those stakeholders naturally want to get involved in decision-making about their tools, and that creates meeting and communication overhead - sometimes to such a degree that stakeholder management and negotiating buy-in ends up being an order of magnitude more work than actually implementing the damn feature being argued about.

To the extent that the amount of work involved in implementing a new feature is inflated by these kind of factors relative to what would have been involved in doing it de novo, I personally conceive of that as "maintenance" work; and in my experience my work on big teams at successful businesses has on average been inflated severalfold by those factors. (I also count work mandated by legal/compliance considerations that arise only after a successful launch as "maintenance". My rough conception of "software maintenance" is that the delta between "the work involved in building a product de novo with the same customer-pleasing features that ours has" and "the work we actually had to do to incrementally build the product in parallel to it being used" as "maintenance".)

Would most people agree with my broad notion of maintenance? I reckon they roughly would, but it's hard to say since people who talk about maintenance rarely attempt to define it with any precision. You give a precise but extremely narrow definition above. Wikipedia likewise gives a precise but extremely broad definition - that maintenance is "modification of software after delivery", under which definition surely over 99.999% of professional software development labour is expended on maintenance! I guess my definition puts me somewhere in the middle.