As a longtime Aperture user, I've known that someday I'd need to switch to a different DAM + editor. Still no problems running Aperture on High Sierra but didn't want to be surprised and decided to evaluate all current options (as of 15 April 2018).
I created a test library of 2017 photos which is approximately 10,000 images. Mostly RAW from a few different cameras, JPEGs from a couple of point & shoots and the occasional iPhone photos. Some have edits, most don't. (BTW, Capture One imported that Aperture library without difficulty.)
Lightroom Classic CC, however, makes a valiant effort but seems to have a couple of problems. First, the process crashes if the computer goes into sleep so changing power settings to "never sleep" took care of that. Now it appears that the import stalls after every 1000 images so this is getting to be a long trial. Seems that the only was to resume imports is to quit the program and start over. Usually that requires the force-quit. Resuming the process then quickly gets to the last image imported and resumes for another 1001 images.
The questions are:
I'd really like to get beyond the basic import and see how the organization suits my workflow, how well it handles RAW conversions and whether I can live with the basic editing functions. Importer just stalled after the latest 1,000 images so I gotta go.