They're not limited to SMS; that's just the minimum viable product. They can do voice and data as well with the same technology. It also should be a lot easier and cheaper for Lynk to deploy than it was for Iridium, since launch costs have gone down significantly (while reliability is higher) and they use extremely small, cheap satellites (at least initially). They are planning on providing a fill-in-the-gaps service and are partnering with existing terrestrial service providers... Providers, at least with the next-generation version, won't have to provide cell towers (in places where there isn't otherwise enough demand to justify the large installation and upkeep costs) purely for gaining complete coverage. This is the sort of thing Iridium was never capable of.

It should be pointed out that Iridium (post-bankruptcy) became profitable and has recently finished launching their entire second-generation constellation (on Falcon 9). Their subscriber base continues to grow.