Bitcoin is not deflationary, it's inflationary, currently at a rate of 3600 new coins every day, this value halves every 4 years or so, so eventually around 2140 there will be no more new coins.
Well, yes. OK, that point was cleared up. But my original question was why not increase the rewards over time instead of decreasing as long as the end limit remains the same.
It'd give late adopters a reason to opt in?
Right, but now you see that why that's not really good, yes? You don't want to reward the later more than you reward the former because you can make a race to be the last in. That would, of course, lead to an absurdity: no one wants to adopt bitcoin first because the later adopters are better rewarded so no one ever adopts it at all. Obviously the better course is to make a race to be the first to adopt and this is the sane thing which happened. You understand this now, yes?
The rewards are increased over time. As long as bitcoin doesn't die, the price per reward in real value terms will always get bigger. A good question would be why the rise is not more gradual instead of block halving why does not the nominal btc amount per block decrease every so slightly every block? This is a good question because block halving is a bit dramatic in the sense that overnight the rewards get cut in half dramatically impacting the profitability of miner set ups.