I think 2.5% is a reasonable and affordable contribution from people making money out of the platform. I would not scale it as someone suggested earlier as a higher percentage in future years seems like gouging, penalising success. Best, I think, to keep it simple.
I think BTW can learn from EOS, all the application(or company) developed and based on the platform should lock a certain number of BTW, in accordance with the proportion of bandwidth. For example, if the application is popular with users and large user accesses result in more bandwidth usage, then the application(or company) should lock a larger number of BTWs, on the other hand, if the application has less users, it can lock less BTW. This will benefit the promotion and appreciation of BTW in the long run.To gain more bandwidth, applications or companies will rent more BTWs, and all BTW holders will be able to benefit from renting their own BTWs.