Before you criticize someone walk a mile in their shoes.
Many years back I was working as a Project Manager on a project which was suffering from scope creep. This was not due to the project team or lack of planning but due to management making promises to the customer over coffee.
As a result the entire team was under duress but one SME was particularly negative during our team calls and meetings. He said bluntly in a meeting "You have a technical background why don't you do it yourself".
My first response was reactionary but then I paused and calmly offered "Why don't you shadow me for a day and if you want I can work with your line manager to position you for a PM role." We did this for a day. I shared all the stuff I was busy with including change requests, risks, escalations, management calls after work and sought his input.
Before the day was finished I could see a change in his demeanor. Anger and contempt replaced with understanding and empathy. At the end of the day he politely said that he was happy being an SME.
People are like icebergs. You don't see most of what they are going through. So if someone is negatively critical no need to react, it is important to respond politely sharing your life and struggles.