Ask HN: Start a web application maintenance only company?

I've been freelancing/contracting for a while now. I really enjoy fixing/improving existing web apps more than creating something from scratch.

Is it reasonable to offer maintenance-only service to small (<20 person) boutique digital agencies even if they have in-house devs ? The benefits I see are:

1. In-house devs will be free to work on new projects rather than fixing new issues

2. I can offer off-hours maintenance so this might be helpful in some cases

3. Team augmentation - fill in for a dev if they are unavailable (eg. vacation)

5. Same timezone (EST - PST work hours) availability and quick turnaround

6. Available by email, slack, phone

wdyt ? Is this reasonable ?

I'm located in Ontario, Canada.