Here is where I see Windows Phone in the next few years.
I see it as a strong business device (something they should have done with WP7).
Windows Mobile from the early days all the way through WM6.5 was a powerful business device. It did lose some business marketshare to Iphone but not that much actually. It was never a great seller as a consumer device though.
Move forward to WP7, it was released with no business or Enterprise features, leaving a huge hole in the market to allow Iphone (who ironically enough licensed exchange support, making it more business friendly that WP7). No current applications or tools from WM6.5 worked on the new device, there was no real benefit to upgrading.
Move forward to Windows Phone 8. It started to add some business features, but still had a ways to go.
Windows Phone 8.1, Added a ton of new business features, and some businesses started taking interest. Some actually have them deployed at the primary device.
Windows 10 is clearly a WIP push to unify the Windows Environment and create 1 core. It is also gaining tons of new enterprise features and security.
As this gets built out, businesses will take notice and it may actually become successful there.
Here is why:
1. Security is strong on 10 once finished (Bitlocker, no side loading, etc).
2. Business (workplaces), this allows custom policies and configurations you could never do on Iphone and Android. This allows the work environment to give access to internal tools and resources without installing apps.
3. Custom App store. Businesses can add their own app store, and due to universal apps can create an app for Desktop, Tablet and Phone in 1 go. The Business private store is separated from the personal store.
4. Dual One Drive support. With a provisioned phone (workplaces), it is possible in Windows 10 to segregate business content and personal content between your personal One Drive and the Business One Drive.
The ability to have a phone with Enterprise class security, custom stores, and access to provision the phone with group policies and segregate business and personal data, will be hugely attractive, and is something Iphone nor Android are doing at this time.
Will the phone ever become a strong consumer device, that is unknown at this time. It may reach that point as more people have them for business, or it may remain primarily an enterprise phone.
Personally the reason Windows Phone works great for me is exactly this.
1. I use it for mail heavily. People talk about unified Inbox, and all these other features. Do you really want a Unified Inbox mixing business and personal mail? Top that off with the fact that you can pin individual mail folders to the Desktop to quickly see when you have important mail based on your Outlook Exchange Rules (something you cannot do on Android or IPhone).
2. WorkPlace, I do use workplace to access internal resources from links in my mail (with full Office support this is awesome). I don't have to pull out my laptop to quickly see a slide deck or update a word/excel document and answer a mail on the go.
3. Internal app store, this is great for getting apps I would not normally have on the go.
4. Continuum, this is excellent for presentations. I can bring up the full slide deck for presentations, and still answer Skype for Business messages (related to the presentation) on the phone. This is a powerful feature.
The list goes on, Windows Mobile 10 is and will become a powerful and useful Enterprise tool that also supports personal use in a segregated environment.