One challenge facing a Product Manager is how to balance limited Development resources between the need for maintenance work on the one hand and for new capabilities on the other. Devoting too many resources to one or the other can cause your product to be out of whack and lose ground to competitors who do a better job of mastering that balance.
With limited resources, it is always a struggle to determine what portion to devote to maintenance over new features. It's a little like trying to fight a two-front war: each front draws precious attention and time, and requires careful consideration of priorities. Too much spent on one front may cause you to lose on the other. And as the need for maintenance grows as your customer base grows, your more mature product is probably facing stiffer competition as its market matures and moves towards commoditization. All the while, you can't afford to neglect your base of existing customers.
Read Reconciling Maintenance And New Releases for a discussion of what you need to consider as you continually strike a balance between development on maintenance releases and on new feature releases.