Full disclosure: I'm a noob at Trading and TA. Have only been doing it for about a year and only seriously learning past 6 months, so I'm not an expert and really I'm only at the stage where I know that there's a lot more I don't know. I've blown up 2 dedicated trading accounts of minimal value and almost fucked my hodl account trading (got called on a LTC/BTC Long when it dipped just before it mooned last year that was not fun). I've taken the time to learn from every mistake I've made, and am only now reliably making money and should continue to do so if I don't make further stupid mistakes (market moving against me isn't a mistake, over leveraging, not managing my risk, and fomoing is). I've invested countless hours in studying and learning trading and TA, and I haven't broken even on my losses yet. Now that is out of the way...
The question about whether TA is ultimately guess work is probably most simply answered yes. However it is informed guess work based on analysing what the market has done and why, and what it is likely to do from here.
Whether you can use TA to increase returns as part of a traditional investing strategy, or as a trader, is highly debatable. Statistically speaking the chances are against you. Generally speaking the average person is better off dollar cost averaging than trying to trade or actively manage their investments. However a small percentage of people do make more money using TA and trading. So that begs the question why, and how do they beat the market? Are they simply "lucky"?
Even if you have the right aptitude and mindset for trading and TA, I think there is definitely an element of luck to it, however creating and sustaining the conditions under which luck can come to you is the challenge. That's where money management, risk analysis, and protection of captital come in to it. In theory if you do those things well, you can be wrong the majority of time and provided you arent over leveraged you can still make money.
People who do TA publicly and don't say there are risks are cunts. There is always risk. Black Swan events, your analysis is incorrect, or wrongly timed, etc. So yeah no one can say for sure what the market will do, only give probabilities. I feel like this was a long way of saying "you have to know what you're doing and it's never a sure bet".