Its not about how hard it is, cuz its not as long as you have the time to play enough towers. You can easily win any tower with any character even if you never touched that characters. Konsumables win you towers by themselves and these are available like candy.
Nobody bought the game to fight against AI for hours every day, so having the unlocks in SP is just a bad design. Especially for people who dont have the time to play AI for hours every day.
They should have made them obtainable via milestones... like play 10/50/100 etc games, win 10/50/100 etcetc games, perform X 10/50/100 times... and so on. ToT could award time crystals instead, which would allow you to bypass a milestone and buy it directly. The shop should offer Time Crystals in exchange for coins/souls/hearts. With all this in place players could unlock stuff without being forced to play something they dont want and at their own pace.
ToT are just repetitive very fast. After you do a few you dont even check the modifier, just pick 3 1 slot projectiles, spam, win. Its as challenging as fighting survival tower on very easy. Furthermore the AI cant handle lots of modifers. And the "fun" aspects fades very quickly when you are "forced" to do a gazillion of them and they are all the same. Only exception are those towers where you team up with other peoples.
P.S. before it comes up... i am not complaining cuz i dont have the time myself. Made top 0,1 every time so far but i know enough people who cant cuz of the time it takes and they rather spend time playing MP.