I play the game daily and i'm sure they're making lots of cash with the game. It'll take you months to get the highest equips without spending some money. Sometimes it just feels like an endless task, especially when the market energy prices go up it just becomes more of a struggle. Buying energy becomes very attractive at this point as it helps you to gain a great boost to be able to do the lowest tears.
Best part of all. Even with the best equips, the game stays really challenging and HARD! This ain't a casual game but it requires mad skills. The challenge that's in this game isn't found anywhere in todays era of gaming which makes me love this game so much more. A run can sometimes take to 3 hours but you just don't pay attention because the game is keeping you on your toes alot. It is very satisfying when you're playing really well as a team.
I just remember the other day when I did a tier 3 run for the first time with "segaismysavior" and some other guild member(s). We didn't make it and gave up so close to the core, but personally I enjoyed every moment.
At some point you will have to buy more equips so you have a great arsenal of weps which you have to pick wisely depending on the run you do. Some people just like to collect weapons for fun. This probably results in alot of money spenders in the game.
Theres alot of stuff to this game which reminds me so much of game design of back in the days. It's pick up and play but as you progress, the game pushes you to step up your game and with time you learn theres more to it than just buttonmashing.
Personally I think Phantasy star can learn a thing or two from SK. It would be awesome if battle required the same tactiful team play spiral knights has.