There has always been competition, but on the other hand, the market has always been divided among three operators. Before, the competition was based on call and text message prices, and now it’s on subscription speeds. There have always been differences in network coverage, which for some may be the basis for choosing an operator.
The ARPU of traditional postpaid subscriptions will certainly be partly reduced by mobile subscriptions switching to fiber. It’s probably more common for more expensive full-speed mobile subscriptions to switch to fiber than entry-level 4G subscriptions. It’s also probably easier for an operator to make device sales as an add-on to a home 5G subscription than to a phone subscription.