In Persuasion, for both the older generation characters and the younger ones, the reasons for marrying remain pretty much the same. The first reason is for financial well-being, the second reason is for emotional attachment.
We do not see the older generation characters marrying a second time because they are already financially secure. For instance, Lady Russell and Sir Walter have no intention of marrying a second time, even though they are a widow and widower respectively. Lady Russell was left very financially well-off by her late husband, so she has no desire or need to marry again. Sir Walter is content to let Elizabeth be the lady of the house, therefore he also feels no inclination to marry a second time (Ch. 1). Hence although romance is still important, we may be able to say that the older generation cares more about financial security at this stage in their lives, than they do about romance.
On the other hand, the younger characters marry for both financial reasons and emotional reasons. Henrietta's relationship with Charles Hayter is an excellent example of this. Whether or not she should marry him is frequently debated. Before she met Wentworth, she wanted to marry him because she was very fond of him. Despite the fact that he is only a clergyman, Henrietta's parents approve of the match because he is the first son and due to inherit land. However, the match is much debated amongst her family members, especially by Marry who feels the Hayters are far beneath the Elliots. and the Musgroves, even declaring, "And, pray, who is Charles Hayter? Notihng but a country curate. A most improper match for Miss Musgrove of Uppecross" (Ch. 9). Nevertheless, if Henrietta marries Charles Hayter she will have satisfied both the need for financial security and the need for romantic love.
Hence, in Persuasion, Austen portrays the older generations as marrying for financial security, while the younger ones still care about both financial security and romance.
Yes of course the institution of marriages has changed through the generations. My parents belong to the greatest generation. When people married in their generation, it was usually for life. My parents have been together since 1939. If there was a problem that arose such as illness, the loss of a job, or infidelity, the couple was more committed to work it out and remain together. If you were divorced or had a child out of wedlock during my parents's younger days, you were scorned and looked down upon. Divorces were hard to get. Nowadays, we dont really take marriage as serious as we used to. Divorces are easier to get and we no longer look down on people that are divorcees. If there is a problem in a marriage as I have already stated in the above, we just say..To heck with it, lets get a divorce. The institution of marriage has definitely changed through the years!!