FWIW I once owned a Takumar lens with a small but very obvious scratch on the front element. After a time i used a black felt tip pens to fill the scratch in - more obvious but it proved to be effective in reducing flare. I am pretty sure that the result of this filled scratch was so minimal that I would have been very hard pushed to have distinuished between photographs taken with this or a similar unscratched lens. Of course this does depend somewhat on lens, ype of lens, subject matter and very importantly any extraneous light, effectiveness of hood used, etc.. But in general, and especially after using some very early lenses with siginificant issues, I would say that poor lens condition is generally rather more forgiving that we often suppose.

Living by the sea and often photographing from boats, I see filters as protection and accept that they need periodic replacement. The filters are rarely immaculate. Resulting images show no discernable degredation much of the time.