The good half is real
One owner tends to mean work was done because something needed doing rather than because a house was being dressed for sale. That is a genuinely different maintenance history, and it is usually the better one.
It also often means the person you are buying from can tell you things nobody else could, which is the exact opposite of an estate sale.
The other half is arithmetic
If nobody has ever sold this house, then nobody has ever had the moment where a buyer's inspection forced a decision. Systems get replaced when they fail or when a sale demands it, and an original owner may have had neither happen.
So the profile is a well kept building with original mechanicals of the same age as the house, which means several of them reach the end of their lives together. Use the year it was built to work out what that means here, because the year is what sets the list.
What to ask, and what it is worth
An original owner can usually answer this precisely, and it is the best question available to you: what has been replaced since you moved in, and roughly when?
Everything they name is a cost you are not inheriting. Everything they do not name is probably original, and if the house is old enough that includes the wiring. Cross check the answer against what is typical for the year, which turns a friendly conversation into a budget.