That would only be the case if night fighters were a simple upgrade. I don't think that should be the case. If heavy fighters are long range escorts and light strike aircraft, night fighters could sacrifice suitability for both for sake of being good as night fighters only. They could even have lower (day) air attack to represent oblique armament.
they wouldnt have to change much. Heavy fighters are less agile than light fighters. If the enemy decides to do nightly bombing countering that with nightfighter versions of your heavy fighters makes alot of sense, while at day they are at an disadvantage when fighting light aircraft. The BF110 wasnt as good as initially hoped due to that agility disadvantage, but turned out to be an excelent night fighter for example.
Don't think they will create night variants for the same reason they will not create winter equipment and such.
You cant really compare Night Fighters and winter eqiupment tho. Winter Equipment doesnt really make much sense, since yes, everybody would use it anyway, it would only really be effective a couple of months a year and having basicly 2 of the same thing with one having slightly better winter resistance makes no sense.
Night Fighters however would be an noticeable addition to the game, to counter night bombing. They have an disadvantage against light fighters, but at night they are excelent hunters. Lets say germany had air superiority and has alot more bf109 for example. If the UK decides to bomb at night, those bf109 will of course fight worse, while those bombers being escorted by night fighter mosquitos for example would help quiet alot to locate and destroy the german interceptors. Same could be done the other way around of course, with germany focusing on missions in the cover of the night if the allies have air superiority.
Simply mass producing regular fighters and adding an "night combat bonus" on them sound like an missed oppertunity. That far we only have carrier based versions anyway, so it really wouldnt overload the air techtree either.