Microsoft indicated it would wait until the release of Windows 10 to announce a new flagship, despite its high-end Lumia 930 dating back to April 2014.
Sony's Xperia Z3 is less old, having launched in September. But the firm had previously refreshed its top model roughly once every six months.
Sony said its focus was on "broadening its customer base" at this time.
The companies' decisions to leave more high-profile launches until later has one immediate benefit: the flagships do not risk being overshadowed by Samsung's Galaxy S6, which was unveiled on Sunday evening.The M4 Aqua is waterproof despite a capless microUSB port
Even so, one expert said the importance of the fast-growing mid-range market should not be underestimated.
"It's much harder to win over people at the high-end than the mid-tier, and it costs more money to do so, especially if you're not one of the top few handset brands," said Carolina Milanese, chief of research at KWP ComTech.
"In the mid-tier there's also less loyalty - consumers switch around more - so there's a bigger proportion of the market to go after.
"And it's easier to demonstrate value, by offering a different balance of price and features.
"But the challenge is that you are competing with more firms and some of them are pretty ruthless when it comes to price."
LG, Motorola and ZTE also unveiled new mid-range smartphones in the days leading up to MWC.Bundled Office
Microsoft is adding two models to its line-up, both of which run Windows Phone 8.1 and will be upgradeable to Windows 10 when it is released later this year.
The Lumia 640 offers owners both 4G data connectivity and ability to fit two Sim cards at once - a popular feature in India, where users often switch network provider when crossing states to get a better rate.
The phone's 5in (12.7cm) screen is bigger and higher resolution than the Lumia 635 it supersedes. It also gains a higher quality eight megapixel camera and an NFC (near field communication) chip for wireless payments and cordless connections to other kit.
The other new model, the Lumia 640 XL, has a bigger 5.7in (14.5cm) screen and a 13-megapixel camera. Both are being bundled with a one-year subscription to Microsoft's Office 365 productivity software suite, including access for an additional PC and tablet.
They will range in cost between ?139 ($155; ?100) and ?219 ($245; ?160), depending on the specification.The Lumia 640 is a dual-Sim model, which may help it appeal to shoppers in India Shrinking appeal?
The Windows Phone platform saw its market share shrink from 3.3% to 2.7% last year, according to recently published figures.
The forthcoming launch of Windows 10 will allow a single app to run across Windows PCs, tablets and phones, rather than having different ecosystems.
Microsoft marketing director Neil Broadley said it would present a "tremendous opportunity" to attract new users.
But one expert said that delaying the release of a new flagship until that point might be misguided.
"The risk is that if Microsoft doesn't release a new flagship soon, its top-end hardware is going to look very old," said Ian Fogg, head of the mobile at the IHS Technology consultancy.
"Having a good flagship can have a halo effect, attracting people to the rest of the range.
"The new phones are upgradeable to Windows 10 anyway, so the fact the operating system is not out yet shouldn't deter Microsoft from releasing a new flagship now."Hundreds of new products will be launched at MWC this year Budget cuts
Sony announced a policy change last month, saying that it was going to limit investment in its smartphone line-up as part of efforts to return to profit.
The Japanese firm's chief executive, Kazuo Hirai, added that he might later consider "an exit strategy" from mobile phones altogether, though he said no such plan was currently in the works.
Releasing the Xperia M4 Aqua may therefore fit Sony's mobile unit better than releasing another tent-pole device packed with expensive new technologies so soon after the last.The Xperia M4 Aqua's marketing materials make a virtue of its ability to be used in water
The 5in (12.7cm) phone's distinguishing features, all shared with one or more other pieces of Sony kit, include:A 13MP main camera and a 5MP front one Up to two days of normal use between charges, thanks in part to a relatively large 2,400mAh battery Waterproofing to depths of 1.5m (4.9ft) for up to 30 minutes, with a capless microUSB port
It will cost about ?300 when it goes on sale.Sony's Xperia Z4 tablet can work under water and stream PlayStation 4 video games
Sony has also announced a new tablet, the Xperia Z4
At 6.1mm (0.24in) thick, it said it was the thinnest 10in (25.4cm) such device on the market.
Dell has a slightly thinner, but smaller model.