Holding back aid to get an announcement on an investigation from a foreign power into a political rival doesn't happen all the time, at least it shouldn't, and he should be impeached for the sole reason as to deter it from happening again, by Trump or any future Democratic or Republican president. If you are going to no impeach you are basically telling Trump, go ahead do it again, do it to the actual Democratic nominee if it happens not to be Biden, and also setting a precedent for future presidents.