The fuselage opening for the door is still present as it’s a common part. The door is replaced with a window panel to seal the space, there is no door there.
Any mechanical component - even if unused - requires servicing and maintenance. No airline would want to pay for that and the fuel cost on every flight to carry a door they don’t want.
The patent was not claiming this was a maintenance issue. They were pointing out that if there were indeed a hidden exit door here that it would still require maintenance, which is why there wasn’t actually a hidden exit door here. Instead it’s a solid panel of some kind.
It's not a permanent welded-in plug, rather it's a door-shaped panel that is secured in using the fastenings where the door would be fastened. That's why you can see the outline.
The one thing an airline likes less than carrying extra weight is not being able to change their minds later :) Or rather, for the option to be available for whoever they sell the plane to after they're done with it.
Why would it need replacing? It's a permanent element, unless the aircraft is refitted in the future when they can remove the panel and fit a door. The door's connection to the fuselage isn't a service item, just the mechanism itself.
Any mechanical component - even if unused - requires servicing and maintenance. No airline would want to pay for that and the fuel cost on every flight to carry a door they don’t want.