I can see why they are saying goodbye
The product was actually a lot more capable than people gave it credit for, in the right environment - i.e. to deploy iOS or manage a small business. For places that had Macs, it filled a gap left in the market by products like Microsoft's original Small Business Server which could be managed by someone in the business; or by small/medium business outside contractors and software developers (including my company). Microsoft gutted SBS (partly because of pressure from the larger contractors) when they decided that everything, that they could force, was going into Microsoft's cloud to give them a reliable income stream.
I guess that Apple looked at the revenue and decided that it was not worth continuing with the amount of resources that it needed. A fair bit of the "server" can be replaced by a NAS, but the integration of Mail, Calendar, website, etc for up to 50 users was more than feasable on a Mac mini (until they cut the number of cores down to 2).