Whether or not Trump should be impeached is an important question independent of its consequences. Therefore, if impeaching Trump means that Pence will become president, that is besides the point if Trump's actions warrant impeachment.
That is all that is needed to be said; however, there are still good reasons to believe Pence would be preferable to Trump even if you object to Pence's politics or his perspective on social issues. First, Trump tried to get John Kasich to be his VP by offering him the power to manage both domestic and foreign affairs, suggesting that Trump will be delegating most of the actual work of the presidency to his VP. This means that Pence is likely to have an unprecedented influence on Trump's approach to governing, so a President Pence may not be so different in policy and actions from a President Trump. Second, some of Trump's most potentially damaging attributes are his unpredictability, his ignorance, and his belligerence, especially in his approach to international affairs. The economy, America's international relationships, and even preventing nuclear proliferation, depend on stability, predictability, and informed leadership. Even if Trump is mostly a figure head, he is likely to do tremendous damage to the economy, which reacts negatively to unpredictability, and America's international relationships, where Trump has mostly unilateral control over US policy. Pence is likely to do less damage than Trump in many areas.