Trillian has product stability and longevity
In its current incarnation (and there have been many), Skype for Business is just a rebranded version of Microsoft Lync, which was an evolution of the Microsoft OCS client, which was itself some weird mutation of the original MSN Messenger client. The latest news: Skype for Business is now being sunset in favor of the new Microsoft Teams, making Skype a dead product walking.
Trillian was first released in July of 2000 with the idea of bringing interoperability to instant messaging. Our simple and sustainable business model - you give us money, we give you Trillian - means you're our customer and not our product, freeing you from worrying about "how we'll make money someday" and what parts of your privacy we'll need to invade to do so. All we've done for the past 20+ years is build great chat software.