Do you think giving every player more free coupon could potentially hurt their bottom line?

If not, why don't you try asking for that.

If it would, then what real difference is there between this suggestion and free coupon for players who want to get ninjas?

On the one hand, you have "I want to spend this other resource on ninja so I can get power with coupon"

On the other hand, you have "I want to spend the new extra coupon on ninja so I can get power with my original coupon"

Remember, whales that buys everything ASAP are not their only source of income. They also get quite a lot from low spender that slowly spend toward the ninjas over time.