Unfortunately this exact type of thing will result in an increase of sales, not a decrease. The reason why Plarium chose fragments specifically to replace the sacred and 5* chicken is because they don't really sell fragments, but they do sell a lot of shards and chickens. If everyone is getting these for free each month then no-one is going to buy them.
Prior to this change, every player was essentially guaranteed 2xsacreds and 2x5* chicken each month through daily logins and monthly/advanced quests. This has now been halved, but the various events that utilize these are still on their same schedule. This means that players that want to continue to participate in these will now need to source their shards and such from elsewhere, with most probably resorting to the shop.
The purpose of the fragments is to distract people from this. Plarium wants us to think that they had to find a way to introduce fragments to the daily logins to show players new game mechanics (i.e. Hex), and that they are trying to give us rewards we want. In reality, they didn't really care what new reward they put in, because all they really wanted to do was remove these 2 items. They are hoping that the shiney guaranteed lego would mask it.
Edit: To try and word this a bit better: Plarium didn't one day say "How can we introduce a new method to give people a free legendary". No, instead they said "How can we remove the sacred and 5* chicken from daily login rewards to make people have to buy them". The motive was the removal, not the addition. They would have simply gone through a variety of possible options to replace these 2 items, settling on the fragments