Darius mana's cost for me it's ok, but then...

http://leagueoflegends.wikia.com/wiki/Darius

These guys can spam their skills all day long from lv 1. Darius has 30 mana cost on his q and w's cost is refunded if he last hit, with a base mana pool of 263 he can spam his q 10 times before being oom. His q, not only provides a good damage ( it's ok) but only a good sustain against an enamy champ (also this is ok), but then why we all did so much problems about champions in top lane without mana, which can rely on a form of free sustain? This of darius is practically a 'free Sustin'? But then why you have to give a cap on sustain of Renekton's q?Do you need to reduce regen of champs as rengar and riven? My answer is YES! It needs, but then why allow sustain as that of Darius at that cost? If we allow him, we should do the same thing we other champs . I think, for example, bringing Rengar's emp w to heal more with a tank build, this would lead this champ in a better spot( or in a less toxic one). Same reasoning with Riven: her shield could scale with life and this would underline her fighter style (and justify all those crowd controls)

I apologize for mistakes, but English is not my first language.