I'm not a huge fan of the fixed subsidy, but it is deflationary long-term. Because the subsidy remains the same while the money supply grows (and may shrink due to lost coins), a graph of the inflation rate over time is steadily downward, and will eventually reach low levels. However, since their stated reason for doing this was to ensure that miners are properly incentivized, it would've made more sense to set the subsidy such that the monetary inflation rate is a constant 0.5% or something. As they've designed it, the inflation rate will start out way too high, and then if grin survives for a very long time, it'll eventually become too low to actually meet their goal of incentivizing miners.
I'm not qualified to make these type judgements but it would seem to me to make a perfect and correct choice on the percentages then you would have to in advance know that the end size of the userbase and security needed to sustain that during the design phase which is impossible. There fore using a best guess that is subject to change will allow you to better see that future as time goes on and make adjustments as that time approaches. This approach will also allow the time for others to weigh in on the design and allow for a growth of those working on the problem as the initial product is being created.
Unlike the vast majority of other altcoins, grin was basically built in the same spirit as Bitcoin - the same ideology. I'm a "Bitcoin maximalist" because I believe in "the Bitcoin idea/ideology", not as part of some game theoretic strategy to make BTC's price go up. If Bitcoin is out-competed in all respects (which grin is very far from doing, and other altcoins aren't even in contention for), then it'd be good for Bitcoin to be replaced by that competitor.
I specifically like this as listening to purists that spout the holy grail code has already been written gets old fast.