They were a wireless pioneer but didn't execute well in the last few years, and right now they just don't have the resources to compete in the age of wireless, convenient, smart, active headphones with feedback mechanisms (which at some point will also be the ones with the most accurate sound rendition, passive dumb headphones have inherent limitations that only smart, active ones can solve).
What Sennheiser is looking for is a company that can benefit from their name, as it's their most valuable asset, and which in turn can provide them with the engineering and manufacturing resources they need. A typical example (perhaps too typical to be true), would be Huawei : they could use Sennheiser's name to prop up their audio credentials as they're starting to develop their lineup of audio devices, while providing the engineering and manufacturing resources Sennheiser could benefit from (as an illustration Huawei is one of the very few companies that could afford to design their own audio over bluetooth chip). Something in a similar vein to "Samsung tuned by AKG" - even though AKG here is just an immaterial brand and represents no team or R&D centre in particular.