With the all the time, effort and resources put in to get 3 key fragments and boxes, I think Hextech Chest shouldn't reward champion shards or at the very least guarantee something other than a champion shard. With the system of getting a champion capsule every level, I think getting only a champion shard from a hextech chest is just a waste of time, effort and resources.
I cannot speak on behalf of other people, but from my experience of playing for days trying to get key fragments (no raging, tilting, int'ing, spamming and so forth) and finally getting enough fragments to forge a key to open a chest, just to open it and get a Graves Champion Shard and nothing else just puts all my effort to waste. In the time it takes me to get all 3 fragments, I would of gained 5 or so levels. Which means 5 champion capsules. Opening the capsules I would get between a min. of 5 champion shards and possible even getting up to 10+ champion shards if I get lucky with multiple shards in one capsule.
With all this in mind, not having done the maths, getting a champion shard that you actually want would be quicker and easier coming from a capsule than from a hextech chest. Eventually at some point players will have all champions unlocked, resulting in capsules only being useful for gaining Blue Essences, leaving players with plenty of essences to buy the new champions as soon as they are released and increasing mastery levels. However with all the levels gained and capsules being opened and shards being disenchanted and plenty of Blue Essences earned between the time of champion releases, this leaves a player with an abundance of BE with all champions unlocked. (As of writing this, I am currently sitting on 91k BE, all champions unlocked and 21 champion capsules still sitting in my loot tab)
With all that being said, a change to getting more valuable rewards from hextech chest or more content being offered for BE ( maybe something among the lines of x amount of BE for x amount of Orange Essences, giving players the opportunities to unlock any skin shards they receive from hextech chest. ) would be gratefully appreciated, giving players who have all champions unlocked and over abundance of BE something to do with these (in my opinion, useless) resources.
But those are just my opinions and ideas.