Nope this is an example to clarify why I made this ruling. Would you make the spell just appear in the spell book. The wizard does not have any ink, not even a feather write with. So how does the spells appear? I am curious about the mechanic of spell writting itself in a spell book.
First, you need an explanation for why you even need expensive ink to copy a spell into a spellbook. Then you can make sense of things from there.
The way I handle it is to treat spellbooks as basically "spell preparation foci", in that same category of quasi-magical tools as spellcasting foci. Because of that, you can't just write text into the book and have it work. It has to be written with special ink, because when you prepare spells from your spellbook you are peforming an at least quasi-magical mental configuration act, and being able to channel the magical symbols through looking at that special ink is part of the ritual of spell preparation.
So with that explanation, how can you write spells into the book
without special focus ink every now and again (when leveling) and have it work? Because it is a spell you have been working on developing since you last leveled up. Every day you are writing bits and pieces of it in there, puzzling over it, testing out bits of it, and effectively infusing it with your own aura/resonance/energy. Instead of spending a couple of hours and using some magic ink to do a quick write, you are investing your extended time and energy into making that spell part of your spell preparation focus. it. It doesn't matter what sort of ink you use in that situation.
Trapped out in the desert? I'd grab a pointy stick and write it in my own blood.
I understand and share the desire not to handwave the spells appearing in the book without the proper ink, but I think there are creative ways to justify it that don't remove class abilities from characters.
As far as putting that in to make wizards less appealing because you'd like to see less of them played... Have you asked your players why they play wizards instead of warlocks and sorcerers? Maybe you'll find out that they really just like wizards, or don't like the others from a conceptual framework. If that's the case punishing them for playing wizards may not be a deterrent until it reaches the rage-quit level, so it might not be the best approach. On the other hand, maybe they are cool with the idea of sorcerers and or warlocks, but they really don't like something mechanical about how 5e did them. (I have issues with sorcerer myself.) In that case, maybe providing positive incentive by altering sorcerers/warlocks mechanically would have better results.