I'm currently looking at a double din Android unit.
Just to clarify some points.
The OS on the radios is unlikely to get updated unless a custom rom is made for it and you apply it yourself. However your chosen navigation app will update as often as you like, so the maps etc will be up to date.
Most smartphones can give a wireless access point, so should you need mobile data on the move you can just wireless connect it to your phone and use the data plan from your phone - no need to buy dongles and extra sim cards etc.
Most of the GPS software is offline, you don't need to use any data - the maps are stored on the memory card in the head unit.
You can use navigation software such as Google Maps which does use data - it downloads the maps as it needs them and also downloads live traffic data to avoid traffic jams and crashes etc. This can be useful but can also eat through your data allowance.
Although the OS will be slightly out of date now and will most likely fall further and further behind it isn't that much of an issue. Many phones/tablets no longer have updates and people still use them without any issues. There are certain security vulnerabilities but these are less of a concern given the fact that the stereo isn't often connected to the internet (dependent on use). You can also add security software to help prevent any attacks. There is also the fact that there probably won't be much sensitive data on your car radio. The most sensitive things will be - email (if you choose to set it up - I won't) and your home address if you choose to put it in to the GPS software (never a good idea with any GPS system, if someone steals your keys and gets in your car and asks the sat nav to take them home it lets them go and break into your house to!)
Someone mentioned a market for a stereo with removable tablet - Xtrons already do one of these, it uses its own tablet rather than one you have purchased before, but it holds it securely in place - it can be removed and there is a basic stereo screen behind which continues to function.
http://www.ebay.co.uk/itm/XTRONS-DOUBLE ... Swk5FUuGjT