I generally agree, although it's not just the disparity in regulations, but that the
disadvantages of not being a licensed taxi are fading.
(You probably know all of the below being in the UK but I thought I'd flesh it out for those overseas.)
Black taxi and minicab operators have been in competition for decades, with regulation finally coming to minicabs about 10 years ago, and an uneasy but mostly quiet co-existence since then.
This co-existence has been underlined by the difference in how minicabs and taxis work. Taxis can be flagged down in the street, taxis can use taxi lanes, taxis can use official taxi ranks at airports and train stations. Minicabs, at least within the M25, are restricted in this regard. They have to be pre-ordered and within perhaps 5 miles of Charing Cross, it's probably a lot easier to flag down a taxi. Minicabs tend to focus on account work, pre-booked airport runs, courier stuff, and suburban journeys.
The problem Uber and things like it introduce is that the friction involved in getting a minicab is almost wiped out. You still have to pre-book, but they have cars milling around everywhere, all trackable, with no surly operators to call. You almost get the taxi ordering experience but with minicab-level drivers and prices. As far as I understand it, the contention of taxi drivers in cities like San Francisco is not so different to this.
So I contend the problem is that "minicab style" services are significantly increasing in quality through things like Uber and even the apps minicab companies are themselves producing (such as Addison Lee's) and therefore taxis are losing many natural advantages.
Having worked in the minicab industry many moons ago, my personal opinion is that this is no bad thing, since minicabs have had a deservedly notorious reputation in the past, and I don't think taxis will die out because there's always going to be a "flag down" market.
What perhaps would redress the balance would be applying the congestion charge to minicabs to help maintain central London as a predominantly taxi zone, and enforcing a toll for minicab airport pickups. It's all about taking baby steps to maintain the uneasy balance that has so far worked for decades. (Or, alternatively, open the whole thing up, shake off 150 years of regulations, and have everyone on a level playing field.. but that won't go down well.)