OSM can never be finished, we just keep improving the map. Our tools and sources get better. We can get better aerial imagery, and this means older data can be a little fuzzy. There's a simple way to improve OSM by looking at old data and seeing if you can improve it.
First enable the TODO list plugin if you haven't already (seriously, it's a great plugin), and enable that window on the right hand side. Then download a large area. The JOSM Continuous Downloader plugin can help here. The we can use JOSM's search functionality to find old data. Let's try to fix up ways which haven't been changed since 2010. Press Control-F to bring up the search box. Then enter
timestamp:/2010 type:way, and press enter. All old ways will be selected. Press the Add button to these to the TODO list. Double click on the first entry to start working on it. Compare it to the aerial imagery, and fix up it's geometry if needed. JOSM's improve way geometry feature (press
W) can make this easier. When you've finished with this way, press
] to go to the next object. Keep going as long as you want. Upload as often as you want.
With regular "gardening" tasks, OSM will be gradually improved and made better.