Living sober as a Christian
The Bible never says we should live simply as Christians. There are plenty of examples in the Bible of godly men that did not live simply. Think of Abraham, Job, Solomon. Could you please give some guidance on this issue?
In some way you are right in that assertion. The Bible admonishes to soberness in many places though: "Wherefore gird up the loins of your mind, be sober, and hope to the end ... (1 Peter 1:13); Teaching us that, denying ungodliness and worldly lusts, we should live soberly, righteously, and godly, in this present world. (Titus 2:12). In other places this exhortations are repeated in different settings or to different groups of people. (1 Thess. 5:6. 8; 1 Tim. 3:23) To live soberly doesn't mean we need to live in poverty or below the standard of living that is common in our surroundings. Christians don't have to live in poverty when everyone around them lives in comfortable dwellings. But also the other way shouldn't be: when there is poverty around us, we should not live lavish. God giveth us all things richly to enjoy (1 Tim. 6:17), however, in the next verse God adds, "that we are to be rich in good works, ready to distribute, willing to communicate." (1 Tim. 6:18). Scripture is always balanced. We may enjoy God's gifts while we also remember the poor, especially those whom He places within our reach.
You mentioned a few Bible saints that were rich. Many could be added, for example, Joseph of Arimathea who gave the Lord his grave etc. Yet with the exception of Solomon, I don't have the impression that they lived lavish or extra-ordinary. Solomon did go beyond what seems necessary. He spent more time in the building of his own palace then in the temple and we can be sure that no expense was spared. Yet this went at the expense of his people who were heavily taxed to finance it all. That's where Solomon went overboard and the people rebelled against his policy after his death.
In Acts we read about the early Christians who shared their 'extra' resources. That is still our calling. We should be aware that whatever God gives us He expects us to become channels of what He gives to others. There are still plenty of people around that have barely enough to survive and that should always engage us to remain sober in how we spend on the unnecessary things that we add to our live. Sure, it is great if you can afford a BMW for your vehicle but if a lesser fancy vehicle is suitable for your needs, then we shouldn't splash out on the unnecessary luxury for that only serves ourselves. Settling for the lesser fancy car and using the extra unspend money to support mission programs that help people with their economic needs would be the Kingdom spirit Jesus teaches.
Lastly, in James 5 we read a condemning portion of God's Word about the rich. James pictures the judgment seat and refers to 'riches that are corrupted (rotten) and clothing that is moth-eaten and gold and silver that is rusted' and they all shall be a witness against you and eat your flesh like fire.' What are all those things? They are the 'extra' that were left behind by the rich when they died. Unused it goes to waste. It wasn't meant to be unused. It was meant to be spend on those that needed it. I fear that many of us are in for some unpleasant surprises on this issue unless we repent of these sins of materialism and hoarding unnecessary riches.
Hope this helps you in your thinking about this subject.