The reason is simple. A new telco needs time to evolve. Even if they have the license and start to accept subscriptions, a lot of customers would still be stucked with their 2 yrs contract. Furthermore, no one will probably want to switch to a new telco and be tied down to two yrs of uncertain network coverage with the new telco.
When MyRepublic indicated that it intends to be data centric and offer cheap data plans, the 3 telcos reacted. M1 started the MySIM plan which is well received and Starhub and Singtel followed. Will these plans eat into the underlying profits ? Yes it may. Don't forget the incumbent also has the large prepaid subscribers which the new telco might not be pursuing.
Lately the share price of M1 and Starhub has taken a beating and has broken multi year low. Will it continue to drop ? I suspect a base of 1.8 for M1 and 2.8 for Starhub,
Once the dust is settled and the 4th telco announced, the share price should stabilize and quarterly reports showing profits, share price will probably rebound strongly. Who knows a 4th telco might be delayed due to economy downtturn. Lastly, M1 might just delist.since the market is full of shorities and traders and not investors.