On the contrary, you get better redundancy by sticking to one carrier and making sure that they really provide separacy though the entire span of the circuit. If you have two carriers running fibre to yoiur building down the same conduit, then you do NOT have separacy and as a result, the redundancy is not there.
Which is the case in about 99% of the commercial buildings providers serve.
Do you have any sources of data to back up that number? I believe that you are wrong and that it is not 99%. In Manhattan and similar city centers, I believe that your view is mostly correct but even there I don't believe that it is as high as 99%. And in places like Silicon Valley or Sacramento, the majority of commercial buildings are surrounded by parking lots, patches of grass and trees. In those areas, if your building doesn't have dual entrance, it is feasible to make it so. Maybe not easy, but definitely within the realm of feasibility.
Service delivery inside of the prem is insidiously complicated unless you understand a little RE and how OSP is linked to the ISP (in side plant).
No argument there. People who want to buy separacy have got to learn all of these things in order to guarantee that they are getting what they think they are getting. Once the market has matured a bit, I expect that 3rd party network suppliers will take over that duty. These 3rd party separacy providers won't have their own networks for the most part, but will do the donkey work to make sure that their carriers are supplying true separacy. In cities like New York and Philly, I would expect this type of third party to put in some of their own metro network links to work around hot-spots where it is too hard to guarantee that the carrier is really providing separacy. This is not unlike the way in which the tier 2 ISPs of the 90's were able to provide better uptime than the tier 1's because they aggregated access to several tier 1 networks.
Im a metro loop, they are almost certainly going to share the same path. It is less certain that they will share a conduit. This is standard if you have the route diversity, which is why you want the provider diversity to make it all work.
I'm not sure what you mean by "path" here, but people who want to buy separacy want those circuits to be physically separate at all points and they want the distance between circuits to be sufficiently great that they two circuits do not share fate. Two sides of the Baltimore tunnel would not qualify as truly separate paths by those standards. This is why I use the term "separacy" rather than "diversity". Separacy seems to have originated among people planning SANs (Storage Area Networks) to interconnect mission-critical data centers in the same metropolitan area. --Michael Dillon