There are now only 2 unmodified 153's in service, that's completely unrefurbished in the original state they arrived from the previous operator. These 2 will be getting full PRM mods in the next few weeks (Including toilets). All of the prm light mods, which is basically all prm accessible modifications except the toilet change are either complete or the final 2 are currently in Landore.
In terms of reliability of the train, the PRM mods make no difference, it's still the same under the carriage. PRM mods are just making them more accessible and easy to use for persons with reduced mobility.
This would make total sense in the short term. However dispensation for these units means they would need to go the PRM light modifications before being allowed into service. Dispensation runs out the end of this month for unmodified units. It's not just about toilets, it's about signage and the ability for persons with reduced mobility to navigate the train.
That would mean more expensive for a very short term solution. The PRM mods take around 3-4 weeks. The program is still ongoing until around the end of January. That means any additional units that would be taken on wouldn't be in service until around March time. To get say 4 units done and in service with mods would likely be around May/June time, which is when the new fleet will likely start going into service, which I can't see TfW justifying the cost.
The current refurbishment state is:
175 - The last 175 is in for refurbishment and should be done by the end of this month
158 - Currently 1 in and I think 2 more 158's to be done which should be done by around February
153 - Currently 2 in and 2 more 153's to get the full mods (with toilet) which should be done around February.
150 - Currently 1 in and a long term ongoing program.
That means by February there will be 4 additional units back in service. The common sense choice should be to postpone the 150 refurb program until the middle of next year