Comment by paxys

Figure out the root cause first.

- Burnout

- Family/personal issues

- Loss of interest in job/company

- Actual incompetence

Have they in the past demonstrated the skills that you'd want for someone in their position? "They've worked here for N years" means nothing. Plenty of people are experts at navigating company politics and showing value to higher ups during performance reviews while doing absolutely nothing day to day. Yes, even for a period of 10 years or longer.

If you truly do want to keep them around, make a plan to get them back on track. Give them work they are interested in. Set and monitor concrete targets.

Otherwise just cut your losses and show them the door. Of course the latter is more of a legal than an engineering question, so reach out to your HR department.