Do remember what overcapacity is though. Simply it's where the supply exceeds the demand. But who knows what the demand will be 10 years down the line? It's extraordinarily difficult to predict demand, but let's face it, it's generally going to increase.
That's essentially why they make aircraft orders with both firm orders and options, as it gives them the flexibility of how many aircraft they actually need when there is a clearer vision in terms of the demand.
Ryanair are not going to bang an extra 400 or 500 extra aircraft into the mix. Airlines and particularly FR and EZY as examples do not just make aircraft orders for expansion although that tends to be the primary reason.
They are also used to replace the older aircraft in order to maintain a young fleet age and in the case of the A320Neo and 737MAX, are more fuel and cost efficient than current generation aircraft. FR have even managed to get 8 extra seats on it's new 737MAX series. These aircraft orders essentially allow them to further reduce their cost base.
This allows them to reduce fares which in turn helps to increase demand, a practice known as "price elasticity of demand" for those who weren't already aware, and the aviation industry does tend to be a very price elastic and price sensitive business.
Also take into account that a good proportion of these aircraft will be used to compete and gain market share from other airlines who reduce their schedules or even go bust.
If FR didn't have this huge aircraft order, they wouldn't be able to renew their fleet and reduce their cost base and they'd be losing market share and essentially be in decline.
FR may have a bigger order by number but remember their fleet size of 300 aircraft is already 50% bigger than EZY's 200, give or take a few.