No-one was ever forced to share with a stranger. You always had a perfectly free choice: share with a stranger and pay less, or have your own cabin and pay more. What's changed is that the former option has been taken away.
From a revenue perspective the option creates a potential weakness: the willing-to-share crowd can gamble on getting solo occupancy for the shared-occupancy rate if they manage to book early. As the train fills and vacant cabins are booked up, passengers unwilling to share can end up having to choose between sharing or taking a different train - and this is especially the case in a market like GB where sharing is the minority choice.
Remove the option and not only does the discount lottery go away, but the experience for non-sharing passengers is effectively
improved because pricing the cheapskates into the seats, or off the train entirely, creates additional cabin availability.
There is a possible "middle ground" of charging solo rate but offering a partial refund if the cabin does end up actually being shared, but this has a higher administrative burden and regardless is unlikely to be popular because it doesn't offer the fixed cheap price.