From a branding and a porfolio perspective you can look at it in different ways:
1. Lumia is a heritage from Nokia so it should disappear quickly in my humble opinion. It feels like a brand created for a transformation period and thus has no longevity and will loose credibility over time.
2. From a parent company perspective, using Microsoft and a sub brand would be too long and create confusion (look at Apple and Iphone for example. There is clear separation from the company name and the product.
3. Surface is a good name but still too generic. I think Microsoft did a great job with the Xbox which explains the product and give good opportunities for sub brands (such as Xbox One etc.)
4. If we play with the name Surface, it is rather generic and probably gives a sense of something bigger than a phone.
5. Therefore, for me, I could see an opportunity to create build a stronger linkage between the products (tablets, phones, computers...) by using a distinct branding for each product category but that still fit with the others.
6. I already have strong brand names in my mind which could work very well together and thus building a stronger portfolio of brands...
7. But for this, I would need to speak directly with someone at MS (and it will be worth while....