I am 68 and have visited 70 countries, not including the countries that no longer exist.
I have lived in Germany for four years and Saudi Arabia for five. I have been retired for six years and traveled extensively since then.
While living overseas in 1981-85 and 1987-91, I traveled with family, including children and did it budget style. Most of the travel was do it yourself, staying in Bed and Breakfasts, etc. Since 2010, my Wife and I have done a mix of ocean cruises, coupled with some land travel, river cruises and land tours. This travel has been more oriented toward paying for tours instead of DIY. However, on some cruises, at some ports, we have DIY due to my research where DIY is more feasible and paying for a tour is just not worth the dollars.
Whether or not you go DIY or with a Tour depends on a number of factors:
1) Age, as one gets older and weather, there is more of a willingness to pay for a tour.
2) Resources, if you are on a budget going DIY might be the only option. Guidebooks can get you a long way.
3) The site makes a huge difference. When visiting Athens, Greece from a cruise ship, it is easy to take the METRO to the city from the port and then to key sites like the Acropolis or Acropolis Museums. Still, if on a tour, the hassle of buying a METRO ticket, getting acquainted with where to go, then walking to the Acropolis is a bit harder. For some, it is worth the extra money. Also, for some age may have limited how far they can walk. On another site, DIY is just not worth the minor savings, if any. We visited Normandy from Le Harve and yes you can do that with a rental car or by train, but the logistics are just too much. The tour we took visited several places, saved us a lot of time. We had too see the Bayeux Tapestry, but also saw the Cathedral, more than one Normandy beach and museum as well as American cemetery.
4)The country makes a difference. Some countries are harder due to language, or the risks involved. For example, going DIY in China is just too much. Chinese crowds can drive you crazy and the language is frightening. A good tour is worth it weight in gold. Also, some countries are more dangerous or the culture is so different. Egypt is a good example. Egypt is a very poor country and sanitation can be a problem, affecting where you can eat or stay. There is a risk that makes going DIY, just not a good idea. Another example of not a good DIY is Moscow. Taking the METRO is not easy with the Russian language it is not easy to determine what station to exit.
5) Last but not least is the tour can provide more details about what you are seeing, far beyond a guidebook. Having a tour guide in the Hermitage Art Museum in St. Petersburg, Russia is great. They know where to take you to see the most important paintings and save you a lot of time with the huge crowds.
Recently, we have found the Free Tours where for three hours a local takes you around to see the high points of the city for no cost but what you tip the person. It is a great way to get to know the city. Also, we have done some great biking tours where you get to see more, since you are not slowed by just walking.
In summary, whether you go DIY or not depends on a lot of factors.