Look around you. Do you see people who have been healed? Yes, you do. How many of those "tithe" to TWI? Vanishingly few, if not to say, none.
Tithing is another issue and healing is not dependent upon it (even if TWI claimed it was).
I'm reminded that Jesus told us to love the lord our God with all our heart, soul, mind and strength, not with just 10% of our being. We are to acknowledge him in all our ways, not just some of them.
Even so, we have fallible bodies: people (including great Christians) still get sick; all die, eventually. What we do have, as Christians, is the peace of God in our hearts, that will strengthen us through all perils, dangers, evils, sicknesses.