Unable to transfer data from my old to my new MacBook Pro using Migration assistant

Migration Assistant really doesn't work well with wi-fi.

The best and most efficient way to transfer data with Migration Assistant is to use the Time Machine backup drive.

Backup your old Mac to an external drive, then connect that drive to the new Mac and use Migration Assistant to complete the transfer.

Back up your Mac with Time Machine - Apple Support

And, by the way, if you aren't using Time Machine or another backup solution, you will eventually lose data. That's a fact.

nicolasAtkins wrote:

Migration Assistant on both MacBooks defaults to WiFi. If I turn WiFi off on the source MacBook, Migration Assistant turns it back on. There doesn't appear to be a way to select a wired connection between the 2 MacBooks.

The only 'wired' way to connect two Macs for Migration Assistant is via an Ethernet cable (so for newer Mac laptops, you'd need USB-C to Ethernet adapters on both ends).

Personally, I keep Time Machine backups of all our Macs anyway (both via a NAS and on SSDs, the latter so I can store a backup offsite). Setting up a new Mac from the TM backup on an SSD is really fast and simple, if the backup is on an HDD it's a little slower but still a lot faster than WiFi.

External storage is relatively cheap, and if you're not already backing up with Time Machine, I recommend you start now. It will make migration easier, of course, but you don't want to be one of the users who join the forum to ask how they can recover data from their damaged drive, only to learn that with current Macs it's not possible and that a backup is the only way. Personally, I recommend the '3-2-1 backup rule'.

Use a wired connection and it will work.