Great thermal picture - great datapoint, too.
I think my Lime V1.2 has the 'acid test' of all cases - - it's a very thin Hammond Manufacturing case as shown in the following picture:
As you can see, the LimeSDR fits like a glove in that case, yet I've run this configuration for many hours at a time and for many days each time it's used that way. I can personally tell you that the LimeSDR gets REAL hot in that aluminum case and the case has hardly any room for air to pass through it (I was looking for a REAL compact case if I was going to carry the LimeSDR for demos and such - - I only put small holes to cross-vent because I didn't have that much room on the panel ends.
I guess the only thing I'm trying to say by all this is that while the LimeSDR DOES get hot - and your pictures really show it - it's an incredible little board because I haven't had any issues with it getting that hot and quitting or failing dead at all - it is resilient..!
But I am in agreement with you and most that some preventive cooling may extend its life out more than we know - and that's a good thing.
Thanks again for posting the pics -
73 de Marty, KN0CK