Well, lets look at what a consolidated slow would look like.
Turgurs is 250 mana for 7.5m duration, 0 resist adjust, 6s recast, 3s cast
Add 60 MR resist adjust for 200 mana
Turgurs II: 450 mana for 7.5 min duration, -60 RA, 6s recast, 3s cast
Adjust the cast time to 1.5 seconds for 200 mana
Turgurs III: 650 mana for 7.5m duration, -60 RA, 6s recast, 1.5s cast
Remove the recast time for 300 mana
Turgurs IV: 950 mana for 7.5m duration, -60 RA, 0s recast, 1.5s cast
Such a spell would not be able to replace a TA in use because the mana cost would be prohibitive. Each of those changes, if implemented, would have an associated mana cost tagged on to the spell (gnomish coders dont work for free

), and they would be significant.