It is always VERY difficult to come in and replace a pastor/teacher (successful or not) who has been at the same church for a very long time where the position has become open because they finally died or because they retired from old age. The problem is often this: congregants either turn on the new replacement pastor:
1) Because they expect him to BE the former pastor! When he fails to live up to their expectations to BE the former pastor they either rebel and split the church or they find an excuse to kick him out the door: "We weren't being fed", "he was not as good a preacher as our beloved former pastor", etc...
2) Another reason this happens is this: by the time the old pastor retired after "hanging on" to his position for years and years (for any number of reasons) his ministry had become stale and unchallenging. The congregation is used to sitting back, putting their feet up on the pew in front of them and not being convicted or challenged to faithfully undertake the ministries God has for them. This has gone on for Y E A R S. Then the new, young, enthusiastic, gung-ho, "Let's ALL get involved and get things done for Jesus" replacement pastor comes in and the congregation is challenged and faithfully preached to on a weekly basis. As a result, many of these "comfortable, complacent 'Christians;" are convicted and challenged to repent, arise from their sleep and to move forward in their faith. Instead of responding to the tug of God's Holy Spirit working through their new minister, they FIRE HIM so that they can pull up the covers, roll over and go back to sleep.
I always advise my seminary and undergraduate students to beware of taking a position where they are replacing a minister who has been at the same church for a very long time and who has finally retired or stayed at the same location until he died. While not impossible to successfully follow and take up the mantle in such a situation (with God all things are possible), the odds are often against the replacement pastor having a successful ministry there and it usually ends badly. Sadly these churches often go through/chew up two or three fine, dedicated evangelical pastors before they finally get to the place where enough time has lapsed between the departure of their former beloved pastor and the "new guy" where they will finally accept the pastor who fills the position.
I hope the above helps to put things into perspective
I think the 1st has a 3rd, more basic component to it.
When a pastor has been at a church a long time, the people left in the church are those who like him, and those who have been drawn to the church who like him. Over time, those who belonged to the church because of the church, rather than the pastor, die of old age or move away.
Anyway, a new pastor comes in, and immediately some of people won’t like him. And those who join churches because of the pastor, and don’t like the new pastor, will obviously leave the church, to find another church with a pastor they like.
Over time, the new pastor, if he is called by God, will draw in new members who like HIM, some of whom may not have joined the church before because they didn’t like the previous pastor.
Churches that change pastors on a regular basis suffer a lot less from this, first because the people who belong to such churches tend to be those who are drawn by church doctrine or congregational relationships more than the teaching/preaching style, and second because no pastor is there long enough to draw a lot of people into the church simply because they like the pastor.
The situation is worse with megachurches, simply because oftentimes the EXISTANCE of a magachurch is due to the pastor, it’s why they grew so large.
I say that because, in the absense of people who just want to be with THAT PASTOR, churches are much more inviting, and do a much better job serving God and Man, if they split and plant when they get to the 500-1000 member range (some say much smaller than that is even better).
By “church planting” using offshoots, God’s work can more readily spread through a community. CHurches spring up locally all around, and each one can offer slightly different emphasis, giving more of the faithful choices that interest them for service.
The most common reason for a church to STOP spawning off sister churches is when you get a pastor that is the focus of the church, so that nobody WANTS to leave to form the sister church.