I wouldn't say CotM is so hard as to only be accessible to hardcore gamers and diehard Castlevania fans. I wouldn't say it's especially hard in general. It just puts up more of a challenge than ridiculously easy games like SotN, HoD, and AoS. Konami probably did make some conscious decision to tone down the difficulty with SotN in order to increase its accessibility, given the reputation the older Castlevania games had, but they went too far, and continued to go too far with later clones of SotN. There are plenty of extremely popular, mainstream games that are more challenging than SotN, so considering how great SotN is in all other areas, I don't think a slight increase in challenge, at the very least, would've hurt its accessibility any, nor would it have hurt HoD's.