I used to work at Towers back in the days when they had the Summer Spectacular show on the lake by the main lawn. Think the show was 6:30pm with a 7pm ride close for all of August, (This was in 1998 / 1999 / 2000 if my memory is right).
All I can say is that as afternoon turned into evening, even on busy good weather days, the park got notably quieter. For the last hour or so, it was not uncommon for Nemesis to be near walk-on, with just the same people clocking up lots of rides.
I know folks on here are theme park / Towers fans, but look at the business side of it. It costs a LOT to keep the park open longer. Most of the seasonal staff are hourly paid, but to keep rides open longer, you also have to pay the retail staff, cleaning staff & catering staff for the extra hours too. Sure, the park will make a bit of money out of the retail & catering outlets being open longer, but you need enough guests staying on park right until the end of the day to really make it pay. A lot of people travel a long way to visit Alton Towers and if they have had a decent ride-fill by 5pm / 6pm, they are thinking of heading home anyway.
To put it into context, back in 2001 on either the second to last or last day of season [fireworks night too] it rained hard, all day. The park got very busy as you would expect on a fireworks day, but the weather alone drove over 10k people out of the turnstiles heading home before the fireworks even started.
Back when I worked there they could extend the park operating hours on the day by an hour if the gate figures were notably higher than expected, or if there had been a significant number of ride breakdowns. I think the decision had to be made by about 1pm so as to give the then staff transport providers [third party companies] the required notice. I believe that Alton Towers now operate a fair bit of staff transport themselves, so in theory they have a bit more flexibility here nowadays.