The following is IMHO of course. Mearls uttered the words 6E and of course this leads to speculation that they are working oin 6E or that 5E is not doing very well.
Now the problem is that online people tend to lean towards extremes. Generally there are posters who think that 5E is the best edition ever (personally its in my top 3, best ever doubtful), and that 5E is perfect and will last for 10-20 years or will be the last one.
Others have been predicting 5E will die a miserable whimpering death. This group is mostly 4E fans who were miffed 5E was not 4.5.
SOme people claim 5E is the best selling D&D ever (its not), but it has probably outsold the other editions since 2003.
Another example would be some posters who think 5E is perfect mechanically where there are holes starting to be found and they have been contradicted by Mearls when they argue things like RAI vs RAW. Off the top of my head there are a few mechanical issues 5E has.
Ok. These can get pretty ridiculous, but nothing a DM can't handle. They could also be deprecated?
I don't know where these are a problem. They seem to do a really good job managing the action economy.
3. Saves at higher levels
What's wrong with them?
4. The way ACs scale at high levels (Monsters start getting +10-+16 to hit, ACs, are still around 20)
You expect ancient dragons to pull their punches?
5. Some classes/archtypes
They can't be supplemented/deprecated?
What's wrong with it?
7. Concentration mechanics
What's wrong with it?
8. Casting multiple spells per round
What's wrong with it?
Some of these effects are underpowered, overpowered, maybe broken, or just complicated.
WotC has indicated they are gonna be moving away from the AP type adventures and that 6E will land when the players want it (AKA when 5E sales decline).
So how long will 5E last.
The bare minimum if they started designing 6E tomorrow would be 6 years. I do not expect this.
In it lasts a decade or more that is great but only 3/7 D&Ds have done this (1E, 2E and BECMI). I would put 10 years at the upper limit. 3E in all its various forms has lasted 17 years, this is coming close to BECMI's 19 year record (if you count Paizo's Pathfinder as 3.x).
No WoTC D&D has lasted more than 5 years in print, 8 if you count 3.0+3.5.
So if I was a betting man (I'm not BTW) I would expect 5E to last in the 8-10 year range. 6E will come eventually when that will be IDK.
Personally I hope that at the 10 year mark they release a new revision of 5 with new rule books that consolidate a bunch of what comes out between 2014 and then.