France still has TGV service on conventional lines.
The LGVs aren't especially unusual or isolated.
In Spain, conventional lines make use of Iberian gauge, while the high speed lines are standard gauge, creating separate isolated lines, needing dedicated stations.
The Shinkansen is perhaps more comparable to Spain in this regard, as the standard gauge Shinkansen lines are completely incompatible with the conventional Japanese narrow gauge lines.
Germany has some Neubaustrecken, which are comparable to the LGV, but the issue there is that the ICE services still use quite a large number of conventional lines.
Berlin-München is 4 and a half hours because the (albeit upgraded) conventional line from Berlin to Erfurt just isn't that fast. The Neubaustrecke from Erfurt to Erlangen is a good bit of 320km/h Vmax line, but it's over pretty fast. The high speed line resumes south of Nürnberg, only as far as Ingolstadt at Vmax 300 km/h. After Ingolstadt, it's back to conventional (upgraded) line.
Germany has an issue where it can build high speed routes - the problem is that it can only do so in patchworks, where the trains constantly change their line speed to adjust to the fact that the line they're on could be 20 or 120 years old.
The TGV also experiences this, especially in southern France, but 70-80% of the journey would be on the LGV. In Germany, only 40-50% would be on Neubaustrecke.