Luka Modric is certainly more successful than Toni Kroos, his title win with Real Madrid is the best proof. Modric has won 24 titles with Real Madrid since joining in 2012, that was only 2 years before Kroos joined. Both players are club legends, but I don't think they will be as widely remembered as Ronaldo.
Kroos and Modric are players whose skills will truly be recognized, even though they will not be remembered like Ronaldo, but still most European football fans will know the history these two players have.
TBH, actually I like Kroos more than Modric but that doesn't mean I say Modric is bad but in terms of skills Kroos has very good skills when in every match he can always provide the right opportunities and some of the great strikers who have played with Kroos will admit that if Kroos has the skill of giving the right ball passes and even Ronaldo recognizes Kroos talent.
so even though Modric joined Madrid much earlier and has won many titles, if given the choice, I would prefer Kroos who should stay.
however, Kroos decision was irreversible and chose to retire and now if Modric had to stay longer with Madrid, perhaps he would be the last remaining legendary player that Madrid has.
UCL match will be the last match for Kroos and all Madrid players will work hard to give the most beautiful gift before Kroos leaves Madrid.