Well against this outcome there is already the rumor that Canon will launch new models early in 2015.
Why would it do that?
To roll out new sensor technology across its line of DSLRs.
Why would it want to do that?
Re-invigorate sales and capitalize on having new technology to sell as a competitive edge against others.
The information in the story doesn't jive. They wouldn't have 2.5 years of stock, especially if they were ramping up to launch and announce a 5d4. I'm not saying it's not feasible to announce another model but they won't have 2.5 years of stock on the 5d3 which is why I question the knowledge of the salesperson in the first place.
Ok, what are the rumors?
(1) That Canon have stopped making 5D3s
(2) That there's a new sensor in the 7D2 that will be announced next month
(3) That there will be follow up new cameras announced early next year.
What don't we know?
(1) How many cameras makes per month
(2) What sort of inventory Canon has
(3) How long Canon thinks it will take to clear that inventory
Maybe Canon have reduced the lead in manufacturing for cameras to be closer to "on demand" and what's required rather than having large amounts of stock, meaning that if they stopped making 5D3s now that they'd be out of stock by Feb-March next year (6 months).
The 5D lasted for 3 years, 5D2 for 3.5 years and the 3 year timeout on the 5D3 is next year.