I think that the problem arises, not with words that we don't use any more (we can easily look their meanings up), but in words that are still in use today, but with different meanings to what they had when the KJV was translated. An example is the word "prevent." Now, that word means to stop something happening. But it doesn't mean that in the KJV. For example:
Ps 88:13 But unto thee have I cried, O LORD; and in the morning shall my prayer prevent thee.
The Psalmist doesn't mean that his prayer will stop God doing something. "Prevent" used to mean "go before" or "come before."
I see this as a bigger problem than obsolete words, because as "prevent" is in everyday use still, a reader of the KJV won't know that it had a different meaning back then.
Ps 88:13 But unto thee have I cried, O LORD; and in the morning shall my prayer prevent thee.
The Psalmist doesn't mean that his prayer will stop God doing something. "Prevent" used to mean "go before" or "come before."
I see this as a bigger problem than obsolete words, because as "prevent" is in everyday use still, a reader of the KJV won't know that it had a different meaning back then.
What you have stated here could definitely be a problem. Regarding the word "prevent," we see a similar example in the New Testament.
1Th 4:15
For this we say unto you by the word of the Lord, that we which are alive and remain unto the coming of the Lord shall not prevent them which are asleep.
1Th 4:16
For the Lord himself shall descend from heaven with a shout, with the voice of the archangel, and with the trump of God: and the dead in Christ shall rise first:
1Th 4:17
Then we which are alive and remain shall be caught up together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord.
Here, the word "prevent" similarly means "precede" in that the dead in Christ will get their glorified bodies slightly before those who will yet be alive at that point in time.
This said, this still need not necessarily be a problem. Again, if one is truly being led by the Holy Spirit, the Spirit of truth, then the Holy Spirit can easily prompt them to look up the meanings of words. I can attest that I have been prompted to do the same many times myself.
Furthermore, in many instances, the newer translations use the same words that the KJV does which had totally different meanings when the Bible was written than they do today, so this criticism is by no means limited to the KJV. One such example would be the word "adoption." In Paul's day, it had a different meaning than it does today, yet many Bible versions still use that same word today, and it has caused a lot of confusion within professing Christendom.
https://biblehub.com/romans/8-15.htm