>and you have staff who know that platform well.

People often ignore how important this is, especially for a small team like Stack Overflow appears to have. A new platform requires training and/or a lack of productivity as employees learn the new system. It requires developer time to convert over any work designed to use the old system. It makes hiring more difficult because now you will either want someone with the knowledge of both your systems which greatly shrinks down your pool of potential hires or new employees will need to learn a new system on the job which increases the ramp time up for any hires. Maybe this doesn't fit the textbook definition of technical debt, but it is still a real and ongoing cost that comes with making a decision to switch tools.