If they are dissatisfied with the aesthetics of the property, they are more than welcome to leave with proper notice. 

These does not affect the quality of life (safety, hygiene, security) and are mainly aesthetics.

My subject line was "Aesthetics important to you?

However, short term, it would be a great touch for tenants, for aesthetics and their heating bill savings. 

If there's a scale from "market/economy" to "design/aesthetics", obviously my default position leans more toward the design side.

Is it simply the aesthetic they tend to dislike?

Their aesthetic quality is debatable.

This was a conversation between my fiancee and I and I sided more towards if its not broke don't fix it attitude and she is more of the aesthetic look of things and future rents of increasing value have more space for the tenants.

I am not a fan of tile aesthetically it's just not pleasing to the eye. 

I don't see it as being unethical to not upgrade unless required or horribly aesthetic and I would say that I percieve I am in the majority when buying turnkey properties.