One possible reason for resistance to a new design is that it's not advanced enough - MAYA - most advanced yet acceptable.
Performance vs preference
What an application needs to be most effective for the business is not identical to what the users like
The user is not always right.
Do your best to take their wishes into account as long as that doesn't compromise other success factors
Reasons the user might be wrong
Learning the existing system is a sunk cost for them
A hard-to-use system might be a barrier to entry for new users, which benefits existing users
They might focus too much on aesthetics, might prefer a pretty system over an effective one
They might have to change the way they do their jobs, and they may not like that
Many people just have a general discomfort with change
Flexibility / Usability tradeoff
Adding flexibility generally reduces usability
Finding the right balance has challenges
Every business application has a core functionality set required for the business to operate
A system that satisfies design principles but doesn't offer this core set is a bad design
Vocal users will usually push for more features
If you suggest a feature to a group of users, somebody will want it, and sometimes they get downright indignant if the feature isn't included in the developed application
Simply adding features to please various people risks bad design
Dramatically new designs make some old features unnecessary, and those features should be ruthlessly pruned
Every feature has cost
Consider both costs and benefits for any feature outside the core set
Development, testing, maintenance time, hidden costs from 80/20 rule, Hick's Law, etc.
get to "good enough"
No design is perfect, you'll never please everyone
Design principles are guidelines, not rules
Separate design principles often have tradeoffs, always be cognizant of costs and benefits
Trimming around the edges rarely yields successful design; You must push into new territory to be successful in most cases
The goal is to get something good, but not something perfect