The Anniversary Event really pulls this into sharp focus for me right now...I really wish SIF played a more significant role in increasing your drops of event-related things like TOT buffs like the Cake buff and pink event items like Cake slices.
When things are on timer its frustrating because, if your main focus in your play session is participating in the event, you know you have a few minutes where nothing can happen because everything's cooling down. It promotes you to just tab out and do something else while you wait....which means as soon as you go back, everything drops for you the next mob you kill, and you're pushed right back out of the game again.
While I appreciate that you can achieve huge amounts of RIF/SIF with boosts, and of course I agree that you shouldn't be able to get event loot and buffs dropping like candy from a piñata (like it was with the buggy Odin chests in Cows that one time), I do wish that you could actively increase your droprate through more active play and through gearing/boosting for drops.
The way I'd probably do it is to get rid of the hard timer and make it a totally soft timer, where the probability of a drop increased through time then went back to zero after a drop. So on average the drop rates would be the same, but you're never totally unable to get another drop - just more unlikely. And since every kill is a chance to drop, killing lots of enemies gives you more chances and thus better drop rates over time, rewarding more active gameplay and not just in-and-out boss killing in patrol zones.
SIF would improve the rate at which the probability "decays" after a drop, but there'd be diminishing returns so that stacking buffs to 1000% SIF might only give you twice as much effect as stacking to 200%.
Math Wonkery
The base drop chance is described by 1-((N-x)/N)^R, where x is the time since the last drop, N is the "safety net" time where you guarantee a drop, and R is a rarity coefficient. That'd give you a curve where the probability climbed very slowly until it got close to the "safety net" time, where it would rapidly increase until it guaranteed a drop. The smaller the value of R (R<1), the slower the probability increases.
SIF, then, would increase the value of R by some function based on log(SIF), so 100% SIF is only twice as effective as 10%, and 1000% is only three times as effective as 10%.
This model, of course, is strongly dependent on kill rate, even assuming boss kills have an innately higher weight for drops. Because you're rolling against a very low probability, you need a lot of rolls to see a reduction in your average drop time. Infrequent boss killing would pretty much guarantee you drop at the "safety net" rate regardless of your SIF.
With an N of 10 mins and an R of 0.001, your probability after 30 seconds would be 1 in 32,000, after 1 minute would be about 1 in 10,000, after 5 mins about 1 in 1500, after 8 mins about 1 in 600, at 9 mins 1 in 400, and at 10 mins its guaranteed. The average time to drop is whenever the probability is less than the amount of enemies you've killed in that timespan. Using places like Cosmic Terms as a guidepost, where you can get 500 kills in maybe 2 minutes, that puts the break-even point somewhere around 5 mins (1 in 1443 chance, average of 1250 kills).
Killing faster than that will give you a better average....if you can get 500 kills in 1 minute, your break-even point drops to around 3 minutes (1 in 2804 chance, average 3000 kills).
Bosses could "count more" than normal kills by dropping with a multiplier to drop chance, maybe 100x better than trash mobs, so that AOE heroes weren't too heavily advantaged. If you're just killing bosses under that parameter, and you're able to kill maybe 2 bosses per minute average in Patrol Zones, generously, then the break-even is again between 5 mins (1 in 14 chance, 10 kills) and 6 mins (1 in 10 chance, 12 kills). That's comparable to pure trash killing, but then of course its also rewarding you for continuing to kill trash between boss fights and not just cherry-picking bosses.