"Speculation" in its normal sense is an uncountable noun and so would not take a plural. In the first sentence, the use of the present perfect progressive form is not natural. Better: "Speculation has lasted for thousands of years". I have to say it's a very long time for any kind of speculation to be lasting! The word "civilisation" would work instead of "speculation". As for the second sentence, it's OK except again use the singular "speculation has been..." "To go on" in its sense of "to continue" often takes progressive tenses.