You may be right, but if you can implement it good, there's no way you can't unbalance everything. I'll give you an example:
Let's say this is the firse 7 day login bonuses:
Day 1: 3 Pokeballs
Day 2: 3 Pure water
Day 3: 5000 Pokedollars
Day 4: 2 Oran berries
Day 5: 2 sets of random seed (1 big, 2 smalls)
Day 6: 2 antidotes and 2 burn heals
Day 7: 1 direct hit, 1 speed x
We can say this is a less offensive balance problem, and the scale can go up without unbalance much from there. Now, we scale consecutive this:
Day 8, 15, 22: Can be 3 of any pokeball, either be superballs, repeat balls, dusk ball, etc.
Day 9, 16, 23: Can scale up to: Soda pop, lemonade or moo moo milk.
Day 10, 17, 24: Can be up any types of money or well be a amulet coin (Since the nerf/buff). Be in mind you recive one of them every month, so they're not intrusive, since you have to constantly log in to get one.
Day 11, 18, 25: Can be any set of berries you can come up with.
Day 12, 19, 26: Any set of seeds. Seriously, any set, don't matter how, as long you will be able to plant at least 2 berries.
Day 13, 20, 27: Any set of stat healing items. (You can put a saffron box at day 27, again, you'll have to log in for 27 consecutive days, so it's a effort at least)
Day 14, 21, 28: Any set of battle stat boosters. (Same here, you can put a lucky egg small, since you have to log in for 28 consecutive days)
Now, once you get to the last big reward, your reward streak will reset, so you'll be back to reward 1: 3 Pokeballs
Also, you can make special login bonuses on holidays.
Noy, you see what i did there? Sonething not so intrussive, and doesn't affect the economy that much. I'll agree with the untradeable items tho.