I disagree. When you MoSCoW ( Must/Should/Could/Would have) the idea the suggestion of a small batch of RA3 locos doesn't stack up.
Although DRS ordered its first batch of 68s in 2012, it continued to have 37s restored until this year. That points to DRS seeing value in type-3 / RA5 locos over 68s which to date haven't exactly been worked off their fleet (wheels!). There are currently 30-40 class 37s on the national rail network (DRS, Network Rail, Colas, ROG - leaving out WCRC because it's unlikely to be in the new loco market) plus a selection of non-reengineered 73s with GBRf, all of which are candidates to be replaced before long. So to the contrary, there should be a market for a lighter, just <2,000hp, ETS, RA5, type 3 loco capable of 90-100mph. Aside from test trains a "new" loco could potentially replace the 68s that are currently working fairly short-formed passenger turns such as Chiltern, Anglia, Northern (and maybe even TPE in the mid-/long-term) leaving the 68s to work the heavier freights for which they were designed. A new type-3 would also be suitable for the nuclear flasks which again are hardly taxing the 68's capability. And they would burn less fuel and have lower track access charges than a heavier type-5, which would mitigate the cost of another type of traction in the fleet. The first order for 68s was only 15 - new type-3 with an initial order from DRS for 10 but with a good possibility of more to follow surely wouldn't be that bad a proposition.