It's been a big week for Microsoft. Announcing its first tablet hardware - Surface - on Monday and then giving developers a preview of Windows Phone 8 two days later, the computing giant would seem to its sights firmly set on post-PC devices.

Of course, that's not entirely true since Windows 8 will run on PCs and laptops too, but the operating system has been designed from the ground up for touchscreens. See also: Windows 8: no touch, no fun.

Windows Phone 8 is undoubtedly a step in the right direction since it provides that crucial incentive for developers to create apps for the OS. Now, instead of a tiny potential market of Windows Phone 7.5 users, the same app will run on Windows smartphones, tablets, laptops and PCs. That can only be good news for Windows Phone owners clamouring for a wider choice of high-quality apps, right? Visit Windows Phone 8 release date and specs.

Unfortunately, probably not. Existing handsets, such as the recently released Nokia Lumia 900, won't be upgradable to Windows Phone 8 and apps written for Windows Phone 8 apps won't run on older devices. There's every chance app developers won't bother to release a separate version for Windows Phone 7.5 owners.

See also: Microsoft launches Windows Phone 8: details

If you've just bought a Windows Phone, you'd be forgiven for feeling betrayed by Microsoft. It turns out that Windows Phone 7.5 uses the old Windows CE core while Windows Phone 8 shares its core with the new desktop and tablet OS. Of course, Microsoft knew this full well when it launched Windows Phone 7, but kept quiet about the fact, presumably to avoid putting off potential buyers.

Effectively, it's a clean break, and makes existing Windows Phone handsets all but obsolete.Given than Windows Phone 7 only launched 18 months ago, it's the second clean break in under two years. For even hardcore Windows Phone fans, that's unacceptable.

It's not good news for hardware manufacturers such as Nokia, either. The firm hasn't sold nearly as many Windows Phones as it had hoped and there's precious little chance of selling many more Lumia 900s now that fans realise they'll be stuck with the software it comes with, albeit with a few tweaks coming in Windows Phone 7.8.

It's the same story for all the other handsets running Windows Phone 7.5. Either they'll have to be sold off cheap or retailers will have to rely on ignorant customers to buy them anyway.

These days we all expect our smartphones (and other devices) to be upgradable. Perhaps that time is over. With Android phones, it's still a lottery whether an update is released for a particular handset. Plus, if you bought your phone on contract, you might be at the mercy of your mobile operator which may take months to roll out the update.

Apple has a gentler approach to obsolescence as it tends to cater for at least two previous generations of its devices before software updates disappear. Owners of the original iPad may not agree, however, as the tablet couldn't run apps such as iMovie despite being only 12 months old, and won't get iOS 6 at all. Those who are still using their iPhone 3GS might have been pleased to discover that iOS 6 is coming until they read the fine print and realised that they won't get most of the headline features. It isn't much better for iPhone 4 owners, who won't get Siri, turn-by-turn navigation nor FaceTime via 3G.

Microsoft says it still cares about Windows Phone 7 users, but offering the same home screen from Windows Phone 8 isn't nearly enough compensation for what essentially amounts to cutting the power to the life-support machine.

From a technical point of view, it's understandable that Microsoft had to make the switch from Windows CE to the new core to make life easier for developers and better for users, but that's no consolation for anyone that's just taken out a two-year contract on a Lumia 900 or even bought one outright. The switch should have happened when Windows Phone 7 was launched and even if the physical handsets couldn't be upgraded to multi-core processors or higher resolution screens, at least the firmware upgrades would have kept them up to date.

When Windows Phone 8 launches later this year, Microsoft had better hope that people don't fear another dead-end OS and hand their money to Apple or Google instead.

 Follow Jim Martin and @PCAdvisor on Twitter.