It's been a little more than a month since Mark Rosewater announced the new block plan. Going to twice-a-year, two-set blocks has been largely embraced by the community in a somewhat unprecedented way—Magic players love to complain, after all.
Rosewater announced this new block plan a year before it was to first be implemented in order for Wizards of the Coast to protect itself from the inevitable backlash of telling players their Khans of Tarkir cards would only be legal for three-quarters of the time they originally assumed. By telling us about the new block plan early, WOTC gets to avoid all that (or at least most of it). Rosewater also included this tidbit:
Change #3: Beginning in 2016, the Core Sets Are No More
The summer of 2015 will have the last core set release, and in 2016, the summer set will be the second set of the block that started in spring of 2016.
HOWEVER! Just like the police, Wizards can lie to you. They're allowed to do it. The criminal justice system allows for deception in the interrogation of suspects, and in the exact same way, WOTC marketing will stop at nothing to deceive you as a Magic player. Always understand that anything WOTC announces about the future is likely buried under seven layers of subtext, double-speak, and outright lies.
There's already something weird about this block: it's based on wedges only for the first set, the draft format is unprecedented, the last set name is currently unknown, time travel is involved, etc. WOTC told us early about the new block plan to avoid angering the community, but once we know how long our cards have in Standard, all bets are off.
So don't be surprised if we get another Lorwyn/Shadowmoor-type situation, or something completely new. WOTC protected its business interests by telling us about the block change, but you can be sure that the company is doing everything it can to preserve its creative integrity and element of surprise moving forward. Could that mean M15 was the last core set? I think there's a good chance.