This way of thinking is strange and evil.
We should use reward mechanism to motivate users, not punishment system to govern customers, although sometimes the difference between the two is not significant.
The problem you mentioned can be solved by greatly increasing the power of the combined card. It is a strange thing that there is no reward for the combined card.
You will be rewarded if you merge, not punished if you don't merge.
How exactly are you planning to do that? Having cards that have 1 attack on level 1 and 10 attack on level 10? Some cards are not really worth to upgrade to max for a single live, I understand that, but you can't just boost the power you get for upgrading cards without completely disrupting the whole system.
It's also difficult to introduce drastic changes like this now, as many cards have already been released and prices have been established.