What 'Signs' Means

'Signs' weren't just random miraculous novelties given in order to make people think that the gospel of the kingdom must have been true.

Signs had that function, yes - but they had that function because they were actual expressions 'of' the kingdom of God itself. 

According to Israel's promises and prophecies in the Old Testament, forgiveness of sins, healing and health, the outpouring of the Spirit and seeing visions and prophesying were always going to be part of Messiah's kingdom. 

So when those very things - which were to be the stuff of the kingdom itself - accompanied the Apostles' claim that the kingdom had come among them, it meant the kingdom really had come among them, and therefore the Apostles' claim about the kingdom was obviously true. 

Like, warmer weather and new sprouts on trees are 'signs' of Summer - not because they point 'to' Summer, but because they 'are' what Summer 'is'. 

So if we say that signs have ceased, that would really mean the kingdom has ceased. And that would mean the gospel has ceased, because the gospel is the gospel 'of the kingdom'. At least in part. 

It would mean that some of the intrinsic nature of the kingdom itself - some of the good news about the kingdom - some of the prophesied and promised story - has been subtracted from.

It wouldn't just mean that only something external to the kingdom but not intrinsically part of the kingdom has been removed - as if like pulling-down a mere road-sign which had pointed in the direction of a city. 

It would mean that part of the character of the kingdom - and part of the message about it - has been altered. 

Consider this quote from George Eldon Ladd, in his ‘A Theology of the New Testament’: 

“The presence of the messianic salvation is also seen in Jesus' miracles of healing, for which the Greek word meaning "to save" is used. The presence of the Kingdom of God in Jesus meant deliverance from hemorrhage (Mk 5:34), blindness (Mk 10:52), demon possession (Lk 8:36), and even death itself (Mk 5:23). Jesus claimed that these deliverances were evidences of the presence of the messianic salvation (Mt 11:4-5). They were pledges of the life of the eschatological Kingdom that will finally mean immortality for the body. The Kingdom of God is concerned not only with people’s souls but with the salvation of the whole person.”