You might not "think" so, but the area has received significant investment. It is currently undergoing a £146m programme of improvements. In the last 30 years, there have been several new/reopened stations, replacement of trains, various improvements to junctions, signalling, etc. It's far from neglected.
That is the signalling improvement previously mentioned. There will be no change to timings. There is no change to reliability. The only difference is the possibility of half hourly mainline services between Penzance and Plymouth - welcome, but not life changing.
What it is is ultimately a cost cutting exercise for Network Rail. They can fire all those signallers in Devon and Cornwall and "off shore" it to Didcot.
The line fell into the sea due to the worst storms in a century, not due to lack of investment/maintenance. The fact that the line is vulnerable has been known since it was built, but major weather-related closures are rare (and are usually due to flooding rather than any structural issue).
Actually, partial closures are common just from sea spray during normal weather. Common enough that the line is bi-directionally signalled (which does wonders for already slow line speeds as trains have to slow or stop).
It might have been a record breaking storm that did it - but the point is that it happened, even though the possibility has been brought up over and over again. Not to mention the floods further up the line.
(and if the GWR couldn't do it back when railways were profitable and labour was cheap, there's little hope of it happening now).
The GWR did want to do something about it. Hitler and his antics put a stop to it. Now, at the very best, we might get an alternative route that is useless other than if Dawlish is closed completely, because it's that slow and low-capacity.
The fact that the 2014 incident is the worst damage that's occurred in the line's ~145 year existence shows that view is justified. There are several other lines which suffer much more regular storm damage (although none of these are "main" lines).
How many of these cut an entire part of the country off from the rest in rail terms? Devon and Cornwall suffers unlike most of the rest of the country in that it has a single rail line going up (except Exeter onwards).