That's certainly possible, plausible even. But it's far from certain.
Broadcast TV has been bleeding viewers for nearly two decades now. The same article this one cites to say Colbert was #1 in its time slot also says, in the last quarter all the late night shows lost 7-8% of their audience. Colbert lost 17% among younger viewers. That's just the last quarter alone. That's a crazy drop.
And remember they aren't just canceling Colbert, but the entire The Late Show altogether. One of their cornerstone programs for I don't even know how many decades.
Also, If they were canceling The Late Show just because Rump wanted them to, he wouldn't accept having to wait a whole year for it. He'd want it now.
So I actually do believe it was a purely financial decision. Colbert's contract probably ends with the the next season, and rather than giving him another raise for a show that's bleeding viewers, it makes sense to just let it all end.