Are you going for brand share OR are you expanding the market? Having a 75% share means that for every 20 people you can talk into coming into the market, 15 of them will automatically buy your product. You can therefore benefit threefold by expanding the market, rather than trying to grab the remaining 25% from whomever has got it.
If you’ve only got 25%, it isn’t going to make much sense spending money trying to grow the market. But if you spend your money just talking to people who are already in the market, and tell them why they should switch to you, you won’t lose those sales.