As already noted, you have to consider whatever long term housing plans/demand there is in the areas. Just because they dont serve anything now, 20 years may be different.
If you are coming out of 3 on the up chances are you arent going to get much above 40mph anyway so its moot. Any faster and a new crossover may not have fitted. Anything on the down into 4 or 5 would likely be facing a red so again not a lot of benefit.
The south end was re-modelled in 2013 which allowed down trains into 4 and 5. Up Goods was upgraded to passenger standard and the up crossover out of platform 3 was upgraded from 15 to 40mph.
Its 75mph through the station, and flashing yellows on the approaches. You might get 10-15 seconds on a good day. Lots of money and disruption for naff all really. Lancaster got remodelled to an extent within the last 8-9 years anyway.