[namelosw]

That's not gonna work for real-world projects. Real-world apps often have larger edits than locking individual cells/cards e.g. Move columns or replace large chunks of spreadsheets in Google Sheets, or Ctrl-A to select all and then drag to move.

Also, if you consider latency, locking does not work well because client B might do operations before he/she even acknowledges the lock from client A because of latency.