Years back some high camshafts were gear driven, presumably chains and belts were preferred for reasons of cost/space/weight.?

Purely for cost and space reasons: a train of gears is expensive to manufacture and must be assembled with precision on the production line, with the entire gear train being indexed during assembly to ensure the valves don't tangle. The gear train also takes up more space, but wouldn't necesserily weigh more than a camchain / sprocket / tensioner assembly.

But a gear cam drive gives more accurate valve timing that is far less likely to change / slip over time, and lowers friction too.

Honda had a debacle with their flagship VF750 V4 bike engine in the mid-80s: large numbers of engines were eating their camshafts within the warranty period. Multiple fixes were tried, from oil additives to new cams but none worked: it put off existing and potential customers alike. Combined with Honda's long-standing reputation for having camchain tensioners on their bikes that were fiddly to set up and often seized, the whole thing nearly killed the motorcycle division.

To salvage its reputatio, Honda redesigned the V4 engine to have gear-driven cams - no tensioner, accurate timing, etc. The press at the time speculated about the sheer design and production costs, but it was worth it, as the engine turned out to be bombproof, and became one of THE bikes of the late 80s and 90s.

It later transpired that the original cam-lunching issue wasn't anything to do with the type of cam drive: it was because to save money on the production line, Honda machined the cylinder head and the camshaft caps separately, instead of line-boring the cam bearing journals as a unit (the latter is more accurate than machining them individually). This meant that some machines had cam bearings that were a bit too loose, allowing the cams to move and wear the bearing surfaces rapidly, while other machines were fine.

But Honda had to do something drastic to salvage the engine's reputation, as it had basically bet the future of the bike division on the V4 engine design.

My impression was that chains were more reliable or longer lasting than belts. Since VW started cutting back on component costs it seems to be no longer true.

A modern aramid belt will probably wear less and last longer than a chain with its continual friction between pins, rollers and plates, especially if oil changes are neglected. In some cars the belt is expected to last the life of the engine. Belts have an unjustified reputation for unreliability because, unlike most chain engines, they often drive other things, usually water pump and tensioner pulley, which can fail causing a perfectly serviceable belt to fry and/or snap. (It nearly happened to me - I changed a whining stiff tension pulley in our old Passat 2.0GL - the belt was still OK when it was sold at 242,000m). Also, the high quality original was sometimes inexpertly replaced during 'servicing', sometimes with an inferior cheapy.

The 5 yearly change advice is a disgraceful sop to the service industry. Fleet and new car buyers don't care because they will offload cars much younger. No significant deterioration will have occurred in such a short time.

Years back some high camshafts were gear driven, presumably chains and belts were preferred for reasons of cost/space/weight.?

Years back, shaft drive was also used.....Bugatti, Auto Union, Sherman tanks!