There is an oddity when running on a heavily competed route : if there is considerable over-supply from 2 or more players, who are all cutting prices to win share, then an additional level of demand opens up, attracted by the low prices : so, the stated demand figure can, in actuality, be much larger : how much is a mystery as its not given and varies depending on how attractive the low price offers are.
As a new entrant on such a route, your RI gives a percentage of the potential pax pool who 'know' about you and so will consider flying with you : an RI:00 still gives a default 20% or so. If the demand level is high enough, you may well have enough potential pax already to fill your plane, especially if the additional demand has been created by your competitors, as you get to access 20% of that too. Basically, your offer doesnt have to convince everyone, just enough to fill the plane, and if you initially fly seats for less than 20% of the demand you can achieve decent loads at decent prices quite easily even with a low RI. As your Ri climbs and attracts a bigger pool of potential pax, you can add flights and/or tweak pricing, as long as your competitors keep their prices low and grow that demand. Effectively, you are piggy-backing their efforts to maximise their share of a growing market. And, more oddity, you can sometimes end up premium-pricing such routes, as it doesnt matter if your prices scare away 80% of the pax as long as the remaining 20% still fill your planes.
On a un-competed route, with demand/supply ratios much less advantageous, your RI has to grow big enough to convince the majority of potential pax, or you have to price-slash heavily to grow that extra demand : thats why getting decent loads in those cases can take so much longer than on a competed route. The difference is, on an un-competed route, the pax are all yours, eventually, on a competed one, you still have to work around what others may do and worse, if one competitor drops out and the other stops price-cutting, demand shrinks back to 'normal' levels and you may find you have to work much harder to keep your loads and prices up.