Alphabetic scripts may be easier to guess the sound of an unknown word (but it is far from straightforward, there are plenty of cases where if you don’t know the word you don’t know the proper pronunciation either), but even guessing the right sound won’t make an unknown word known. That is what dictionaries are for.
There isn’t any fundamental difference between English and Japanese (or Chinese) in that. The difference is that instead of searching in a dictionary by alphabetic order of letters (note: you don’t search by sound, it may be hard to find an unknown English word you heard without seeing its spelling), you search in a dictionary by kanji classification (radical, number of strokes, shape) - or, on tactical input electronic devices you can just write the unknown word with your finger, even easier.
Actually searching by kanji is quite easy once you are acostumed to it.
I often use the dictionary. A couple of days ago I wanted to use the word 爪, I knew the meaning and writing, but wasn’t sure of the sound - and it was for an oral presentation -, so I looked it up.
I also read in a book 棲んでいる, it had furigana, so I saw it was すんでいる、but wasn’t sure of the meaning (after all, if it was written with 棲 instead of 住 there may be a reason), so I looked it up.
Then there are people’s (or places) names… that is indeed an upper level, and usually they are not in general use language dictionaries. But on our age we are quite lucky, so I searched in internet for 新美南吉 and got not only the sound of his name but also information on his life. Now, if I saw his name again, I know how to read it, because I know whom he is.
It seems from your messages that you think being able to said correct sound for unknown words is the same as “reading”.
I know another language, for which there is an ongoing heated debate on how to write it. There is one side that says the writing must reflect closely the sound (which leads to writing fragmentation due to dialect variations, and in the end greatly reduces the geographical spanning a written text may cover). And another side that says the language being the same, the spelling should be the same (which leads to the reading or writing of unknown words not being directly guessable, but had the advantage to maximize the audience of a given text).
I once asked one of the proponents of the first way what he thinks would be better, a foreigner being able to utter the correct sounds, with no idea of what those sounds mean; or a native going to the trouble learning writing rules but being able to write to, and read from, all people knowing those rules, regardless of oral, accent and dialect variations.
He explicitly told me the first option was desirable, which for me means he doesn’t see that language as a tool for communication (and, indeed, he usually uses another, different, language to communicate, even with natives. He only uses the native language for writing poetry for a small local audience).
That language uses, in both cases, a roman alphabet. So, that dichotomy exists even for alphabetic languages. A logographic writing just makes it more visible (pun intended).
thousands of kanji
Kanji are not like letters (kana are).
Kanji are made of components. Kanji are lique words, and components like letters.
even seeing 棲 for the first time, it is easy to look up, as I can see 「木妻」、「木一ヨl女」、「一lノ\一ヨlくノ一」
There is even an alphabetic script that groups letters on kanji-shaped sillabe clusters. While there are thousands of such hangeul clusters, the set of letters is actually limited (around 30 iirc)