Could I interest you in an adjective? In fact, 'nice-to-have' is already an adjective used substantively.* That is, it modifies a (vaguely) implied noun, just like slothful in this passage from Proverbs 24 about people who consider maintenance merely nice to have:
I went by the field of the slothful, and by the vineyard of the man void of understanding, and lo, it was all grown over with thorns.
Implied nouns in situations like these are usually pretty vague and don't add much to the meaning, which is why people prefer to omit them. In your sentence, the implied noun is something like 'thing', and in the passage from Proverbs, it's something like 'man' or 'person' or 'people'.
Regardless of the part of speech, it sounds like what you want is a modifier for 'maintenance'. So here are some options:
Maintenance isn't just nice, it's necessary.
Maintenance isn't merely nice to have, it's a necessity.
Maintenance isn't optional, it's mandatory.
Maintenance isn't nice, it's necessary.
I find the last one the most forceful, but of course your exact word choice should depend on exactly what you what to say, what connotations you want to evoke, and where you want to place the emphasis. The last one implies that maintenance is an unpleasant chore: maybe you want to empathize with the reader's dislike of maintenance, or maybe you'd rather not suggest that at all.
If you really want a noun, you could make the implied noun explicit. This suggests a stronger verb than 'have':
Maintenance isn't merely a nice thing to do, it's a necessity.
*A compound adjective containing a noun and an infinitive verb, but let's not split hairs.