It's called product cannibalism.
Free or not, continued support of 4Ed could eat into sales of 5th, just as continued sales of 3.X could have eroded 4Ed sales.
In the TSR era when you had two different print products sitting on the retail shelf for two different campaign settings, sure. Consumers who only care about a certain product line, only buy that one, but TSR still had to bear the cost of producing, warehousing, and distributing two separate product lines.
That is why product cannibalism killed TSR.
In the internet age, this is no longer true, as long as a single DDI subscription gets you access to BOTH 4e and 5e.
Then you keep the 4e players who want a DDI sub for 4e, but don't care about 5e AND you get potential business from the 5e players who don't care about 4e but want 5e.
Since the 4e products are pretty much in archive mode, there is little additional cost simply to provide access to existing content, and a lot of potential revenue to be made. Plus the access to 5e content doubles as marketing for that content that may get those diehard 4e players to maybe dip their toe in the 5e waters.
Hell, I think you could extend this strategy to 1e, 2e, and 3e, as well. Granted this can be a little riskier, since DDI style content doesn't already exist for those editions, which means there is a now a cost with no guarantee of added revenue. But at the very least, I would offer the back catalog of prior edition products for sale.
Heck, if DDI offered even online only access to WotC's entire back catalog of D&D books and adventures, plus all the new 5e content as it comes out, I'll just give WotC access to my bank account and they can bill me for life.