I think the reason you have to keep upgrading is because you are moving to another location and cannot bring the upgrades with you... (my two cents) :D. Still fun game...
Survivail mode is indeed difficult to a high degree of Crazyness... but there is probably a small chance of luck and being able to evade all the bullets just enough to get wave 20...but i have not gotten their yet XD. Just takes practice.
To all that keep asking about the wave numbers... when you get one of the crates it drops your wave number...inturn giving you two TP per wave so stop complaining.