According to
Wikipedia, Pathfinder first outsold D&D in Spring 2011, while 4E was still being actively supported. To be fair, some commentary at the time (like
this ENWorld post) similarly attributed this to a slower release schedule... but it seems that reduced release schedule was close to the one in the earliest years of 5E, which easily trounced Pathfinder. Also of note: according to citations in that linked ENWorld post, Pathfinder and D&D were neck-and-neck even earlier than that, in fall 2010.
Now, I would agree that once D&D Next was announced in January 2012, it would certainly send 4E sales into a death spiral, even had support continued in any serious way. But it looks like Pathfinder was out-competing them well before that, before support had truly disappeared.
YEs you are correct, it was the slow down of 4e with several announced products getting cancelled. Why this is different to the earlier D&D release schedule for 5e is you are comparing a dying product (4e) line to a new, hot product line (5e). 4e was in a tailspin once the release schedule slowed and the follow up products for Essentials were canceled like the Nentir Vale Gazetteer, the Ravenloft Campaign Setting etc. It was writing on the wall and the tail end of 4e was edition neutralproducts so active support is a misnomer. Halls of Undermountain was the last pure 4e product in 2012, April 2012. Up to Undermountain book releases had slipped to every other month and PF was releasing new books every month whether it was a rules expansion, a Golarion chapbook etc. that drummed up sales. Undermountain was the last PURE 4e book and then it was all setting neutral with Menzoberanzan being the first of those while still having 4e style trade dress and Ed Greenwood's FR dropping the pretense altogether as the very next release. Everything for 2012 was edition neutral like materials such as dungeon tiles and cards for their failed random card product line.
2011 didn't see a new book release until Heroes of Shadow on 4/19, a full five months after Essentials launched and before Essentials it was multiple major releases a month. 2010 had, starting in January: Underdark, PH Races: Dragonborn, February: Martial Power 2, March: PHB 3 and Hammerfast, April: Plane Above, May: Slaying Stone, Strategy Guide, Dungeon Mag Annual, June: PHB Races Tiefling, MM3 July: Demonomicon, Tomb of Horrors, Vor Rukoth, Orcs of Stonefang Pass August: Dark Sun Creature Catalogue, Psionic Power, Marauders of the Dune Sea, Dark Sun Campaign Setting, and then August was the launch of Essentials and no real heavy releases followed. The next major release after the 3 month roll out was Heroes of Shadow. Essentials last release before that, as noted was: Heroes of the Forgotten Kingdom on 11/16/2010. Then Heroes of Shadow was on 4/19/11.
5 months with no major releases. AFter Heroes of Shadow came... Gloomwrought in May, Threats to Nentir Vale Monster collection in June, Neverwinter in August, Gardmore in September, Heroes of the Feywild in November, BoVD in December, Heroes of Elemental Chaos in February 2012 and Undermountain in April.
That is a very
steep drop off in support in comparison to Pathfinder's robust support on a monthly basis. The long gaps are the months where PF would overtake D&D for a month or so and when a new release came out D&D would take back the top spot. There was a short time where it seemed like they were going to start ramping support back up and then the rug was pulled out in January 2012 when "Next" was announced. Ravenloft, which was to be a "new" rpg was cancelled as early as 2011 when Mordenkainen's Magnificent Emporium was originally cancelled along with the Class Compendium and the Hero Builder's Handbook. Mordy was eventually released after the close to finished products were sorted out and rescheduled. According to Applecine, Essentials officially ended in 2010.